More on the sources of Manticoran wealth
Your analysis of how Manticore fits into the commerce pattern of the explored galaxy is largely accurate. In addition, however, it should be noted that by way of Beowulf, Manticore is within very easy reach of the Sol system itself, while the network of wormhole junctions which it controls or to which it has access give it incredible reach both around the perimeter of the League and deep into it, as well. Your analogy of all of the major Earth shipping canals and bottlenecks is particularly apt in that Manticore is the one star system through which virtually all of the wormhole networks interconnect. Without the Manticoran Wormhole Junction, the interconnectivity which makes it possible to reach all the way around the perimeter of something the size of the Solarian League in literally only a very few weeks would not exist and much lengthier voyages through hyper-space would be required to tie the other junctions together.
I believe that quite some time ago I commented on the sort of "situational awareness" this position at the literal center of the explored galaxy's trade routes provides to Manticore where questions of technological innovation are concerned. I think, though, that some people are missing part of the point of what I was saying at the time largely because Manticore is missing it, as well.
The entire explored galaxy has been accustomed for a thousand years or so of thinking of the Solarian League as the preeminent political, social, industrial, economic, and technological center of the human race. Within that concept of the League, there are certain star systems which are considered to be preeminent in specialized areas -- I'm speaking here, specifically, of Beowulf in the bio-sciences, although there are other star systems which haven't been mentioned in the same way (because of distance from Manticore and/or the fact that they just haven't been particularly significant yet in the books) that are seen as equally prominent leaders in other fields. Unlike most of the League, however, Manticore has ready access to virtually all of those leaders and trendsetters. It can observe what they're up to, bring that information back home, and build upon it. Again and again in the novels I've had characters thinking about the fact that "Solarian technology is the best money can buy" or reflecting similar attitudes. Even Manticorans tend to think this way. The truth, however, is that Manticoran technology, virtually across the board, is actually superior in almost every category to Solarian technology.
Now, before everyone starts sitting up and saying "What? Ridiculous!" let me explain a little bit about what I mean here.
In many respects, the Solarian League in the period which produced the Star Kingdom of Manticore as of Honor Harrington's time, is very different from our own experience of technology and innovation. It wasn't just warship technology that plateaued over the centuries before the Havenite Wars. It was much more noticeable in warship technology and doctrine (especially in the novels, given the characters' focus on a looming war for survival), and the plateau didn't apply across the board to all areas of technology or of research and development, but there was not the level of innovation almost for innovation's sake which is part of the mindset in, say, the United States or Japan of today. In our own experience, we've been doing the "frenetic technological advancement" thing for only a couple of hundred years, at the outside. In the Honorverse, they've had literally a couple of thousand years of technological advancement, and the rate of change and advancement dropped off quite a few centuries before Honor Harrington's birth. Nor did it really matter very much that it had. The technological capabilities which existed were more than adequate to meet the perceived needs of the Solarian League, and where needs were unmet, it wasn't because the technological capabilities didn't exist, but because those capabilities were not deployed to the areas where needs weren't being met. That is, systems in the Verge, for example, which were too poverty-stricken or had been separated from the mainstream for lengthy periods of time didn't have access to technology, and hence could not profit from it. As a result, in a bizarre sort of way, some of those star systems (I might mention Grayson, for example) were actually much more innovative than the Solarian League, despite the relative crudity of the technology available to them, precisely because they had to figure out ways to meet their essential needs without that mature, plateaued technology.
What all of this meant in terms of technology, industry, etc., was that the League actually represented an example of continually improving technology but technology which improved very slowly by our own present-day standards. Innovation was rewarded, but not anywhere near so amply as our own experience might have led us to expect that it would have been. Even planets and star systems known as leaders in their fields were much more "bureaucratic-minded" in their approach to basic research, looking for refinements of existing technologies and techniques rather than continually pressing towards new frontiers.
The Star Kingdom of Manticore, however, always represented an example of a star system which was bucking that particular tide. From the very beginning, when it was necessary for the cryo-transported original colonists to "catch-up" on everything that had happened technologically during their lengthy voyage, Manticore has been extremely "forward-looking." The need to import essential specialists following the plague years, and the fact that the Star Kingdom tended to attract so many relatively young, very good specialists, also contributed to that mindset. When the wormhole junction was discovered, and Manticore began transforming itself from the equivalent of the Kingdom of Denmark into a first-rank interstellar power, that forward-looking mindset went with it, and was then amply fertilized and cross pollinated by the Star Kingdom's exposure to the best the League had to offer in technology on such a vast scale. Moreover, in addition to its towering status in the carrying trade, the Star Kingdom has been a logical industrial hub to help serve the needs of the stupendous market its merchant marine serves, and even before the People's Republic of Haven emerged as a distinct threat to the Star Kingdom's sovereignty, the Manticoran need to protect its merchant shipping provided an extraordinarily powerful impetus towards the development of a strong and effective Navy and the infrastructure needed to support it.
I said that your analogy in comparing Manticore to all of Earth's shipping canals and bottlenecks was apt, but, in fact, it falls well short of describing Manticore's position because there's never been anything remotely like it in our own historical experience. I mean that literally. No terrestrial nation-state has ever had such a commanding advantage of position where Earth's sea routes are concerned. For one thing, it would be a physical impossibility for any one nationstate to be placed in that position on a planetary scale. Not even the position of Great Britain in the period between the American Civil War and World War I comes close.
Had any single nationstate been in a position to attempt to exert the sort of control and influence Manticore routinely exerts, it would undoubtedly have found itself fighting for its very life against all of its competitors. In Manticore's case, however, that didn't happen until the People's Republic of Haven began its career of conquest, and it didn't happen for several reasons. One was that Manticore took great care to protect itself against anything of the sort. (In that sense, King Roger's policy of increasing the Star Kingdom's fleet strength in the face of the Havenite threat should actually be seen as a continuation of traditional Manticoran policies, although the anti-Navy political parties tried to portray it as an alarmist overreaction.) Another, quite frankly, was that while the Star Kingdom's position was busy making the Star Kingdom's population indecently wealthy, no one else who would have had a realistic shot at taking the wormhole junction away from Manticore, or conquering Manticore outright, really felt any pressing ambition to do anything of the sort. Individual elements and factions within the Solarian League -- like ambitious OFS governors or ambitious shipping lines -- would undoubtedly have enthusiastically supported some sort of filibustering expedition against Manticore, but the League as a whole certainly did not (it already had everything it wanted, after all), and nothing much smaller than the League (or the People's Republic) could have had any hope of taking the Star Kingdom. Manticore's close relationship with Beowulf, and its proximity to the Sol system via the junction, also played a major part in the Star Kingdom's ability to both survive and maintain control of the stupendous prize astrophysics (and your humble servant [;-)]) had given it.
The fact that the Star Kingdom has managed to survive, to formulate policies and military postures which have permitted it to retain its grasp on the junction, and to prosper financially because of its domination of the carrying trade, has also propelled the Star Kingdom, without its even having realized it itself, into what is quite probably the position everyone else thinks the Sol system holds. The figures for the Star Kingdom's gross system product as a percentage of the Sol system's gross system product were reasonably accurate, although a bit conservative, before the First Havenite War. No one has really attempted to recompute where Manticore is vis-à-vis Sol now (1921 PD). If they did, and if they managed to do it accurately, they would probably receive a severe surprise.
Despite the expenses involved in fighting the People's Republic while simultaneously financing a revolution in naval technology, strategy, and tactics, the Manticoran economy has grown at a prodigious rate over the last quarter-century. Obviously, the internal investment in shipbuilding (especially warship construction), attendant industrial expansion, and all of the other government expenditures which inevitably result from having to finance a major war for a lengthy period, are going to have a significant impact on the amount of cash flowing into the local economy. At the same time, however, usage on the previously existing wormhole junction routes (aside from the Manticore-to-Haven routes) has increased both steadily and heavily, and the civilian manufacturing sector has increased right along with it. The addition of the Lynx Terminus, and the way that it taps into an entire network of wormhole junctions which were previously outside Manticore's reach is already having a tremendous multiplying effect, as well. The reason the entire debate over the Lynx Terminus (as opposed to the Talbott Cluster is a whole) was framed the way that it was in the Star Kingdom and received such enthusiastic support from virtually all sectors was that it effectively offered the final piece needed to give the Star Kingdom's merchant ships what amounts to very nearly 100% penetration of the Solarian League's shipping lanes by tying into those previously out-of-reach wormholes. (And it's worth noting here, I suppose, that even if something happened to the Solarian League as a political unit, the planets with which the Star Kingdom is currently trading would still be there, and the existence of the wormhole networks would make the Manticoran routes even more attractive because of the additional security against privateers or pirates those "shortcuts" would provide. In other words, a collapse of the Solarian League could actually improve Manticore's economic position. [Insert "Tum-te-tum-te-tum" here.])
Because of all these factors flowing together (and let's not forget the Grayson example and influence), Manticore is very much in the innovative, cutting edge mindset which the vast majority of the Solarian League isn't in. Manticore has attracted to itself what is undoubtedly the finest single R&D community in the human-explored galaxy. People who love research for the sake of research find themselves being actively recruited by Manticoran civilian industry, as well as the Manticoran military. They are invited into what is by any standard the wealthiest single star system (as compared to multi-system political unit) in the history of mankind, where they receive the sort of rewards and public approval they don't get elsewhere. And once they enter that star system, the majority of them (not all, certainly, but the majority) also discover that living in a political system which is not run by vast, entrenched, out-of-control bureaucracies is far more to their taste than wherever they came from.
Horrible Hemphill's achievements over at BuWeaps are not solely the result of the fact that the Royal Manticoran Navy started doing research sooner because it recognized the need for a qualitative edge over the Havenite juggernaut. Rather, they are the achievements of a forward-looking military policy (initiated by Elizabeth's father), bolstered by a multi-generational, society-wide cultural bias in favor of innovation and improvement, and funded by an incredibly wealthy economy which recognizes that its preeminence and ability to sustain its position and wealth require it to remain the focus of applied innovation and maintain its entrepreneurial bias. (Let's not overlook the extent to which Willard Neufsteiler's reflection of those attitudes combined with Honor's position on Grayson to create Grayson Skydomes, which is now a serious contender for the number two spot behind the Hauptman cartel within the Manticoran economy.) This particular combination of motivations, opportunities, and challenges explains why the Star Kingdom of Manticore has been able to accomplish all that it has accomplished.
The main reason that the Star Kingdom never previously expanded physically was that there was no need to. Population pressure was never a problem, it had greater wealth and industrial power concentrated in its single binary star system than virtually any other non-League star nation, and it had a powerful anti-imperialist political tradition. (In my own opinion, "anti-imperialist" probably isn't the best possible term, since Manticore saw its role as the vociferous champion of interstellar free-trade in no small part as a means of protecting its position as the galaxy's preeminent mercantile power. I think that could very reasonably be described as a sort of economic imperialism, although Manticore never really wanted to control anyone else's organs of government or financial policies, since there was no need for it to do so in order to safeguard its own position. When I use the term "anti-imperialist in this little essay [g], it's because I can't come up with a better one, and I am using it specifically in terms of a philosophy opposed to the territorial expansion of a nation.)
In addition to the factors which prevented Manticore from needing to expand territorially, there were many factors which actively mitigated against expanding. From an economic and a military perspective, it seemed likely that more territory would simply have increased military responsibilities (to protect that territory) without a commensurate increase in economic clout or military manpower, at least in the short term. From a political and a social perspective, Manticore has recognized for generations that its unique position -- in terms of personal freedoms and opportunities, as well as economic and military power -- results as much or more from fortuitous accidents (like the mere existence of the Manticoran Wormhole Junction) as from the political astuteness of its statesmen and political philosophers. Students of the Star Kingdom's history are particularly aware of this, having watched the way things worked out over the generations of political evolution, but the vast majority of Manticorans are almost instinctually aware of the fact that the socioeconomic and cultural amalgam which underpins their political system and economic successes is unique. The thought of fooling around with that amalgam, of potentially destabilizing it, or spreading it too thin, or bringing in too many people who don't understand how to make it work, is legitimately frightening (and helps to explain why the shenanigans of the oligarchs at the Constitutional Convention in Spindle were such a threat to the entire annexation effort).
The consequences and challenges of fighting a war for 20-odd years have had a profound effect on the traditional Manticoran anti-imperialistic mindset, though. There's still a great deal of concern about how one can possibly go about integrating all of those "alien" populations into the Star Kingdom's political system without destroying that system in the process. (In some ways, this is an echo of the concern of the convention which drafted the Star Kingdom's constitution following the plague years, and that irony is not lost on many Manticoran political thinkers. Not that recognizing the resonances makes the problem any more tractable.) Despite that concern, however, there's been a sort of general recognition that the Star Kingdom needs more "strategic depth." The real turning point was undoubtedly the partitioning of Silesia with the Andermani Empire (thus, by the way, validating, if only after-the-fact, the Havenite view that the Star Kingdom was in the process of turning imperialist), although one could make a very strong case for the process actually beginning with the decision to formally annex the Basilisk system following the People's Republic's first attempt to seize that terminus of the Junction. When the Talbott Cluster came along requesting annexation, the debate was no longer over whether or not territory would be annexed, since that had already happened in Silesia. Instead, the debate was over how that newly annexed territory (and the people living in it) would be fitted into the mosaic of the Star Kingdom's traditional political structures and processes. And that was the real reason the oligarchs' foot-dragging at Spindle was so nearly fatal to the entire annexation, since it seemed to augur that the answer to the debate was "not very well." On the other hand, it's also the reason that the pattern established in Talbott is so likely to be central to the Star Empire's future existence, since it will undoubtedly be the precedent for any future multi-star additions.
Ultimately, it seems highly probable that the Talbott Cluster -- and Silesia -- will become extremely affluent under Manticoran aegis (assuming that Manticore survives at all, of course), but there's virtually no way that those areas could possibly attain the local density of economic and industrial muscle which the Old Star Kingdom has enjoyed and, presumably, will continue to enjoy. There is, quite simply, only one Manticoran Wormhole Junction, and it can't be moved around the galaxy. But while the rest of the Star Empire probably won't reach the pinnacle of wealth the Old Star Kingdom already enjoys, it is entirely possible -- likely, in fact -- that the Manticoran focus on growing local economies as rapidly as possible while simultaneously introducing the Manticoran traditions of personal liberties and direct participation in a working political system (free of stultifying bureaucratic underbrush) will produce a standard of living for the Star Empire as a whole which will at least match and probably exceed that of the Old League's core worlds.
Assuming, of course, that the Star Kingdom survives.