And you _still_ don't explain why the Guard's return to Earth is limited by Dahak's speed. Why not send 1 Guard ship ahead at hyper as soon as possible?
1 Guard ship could make the journey to Earth, defeat the scouts, pick up a huge load of technicians and return to Bia before Dahak completed the trip from Bia to Earth, in fact, the Guard ship would be just a little behind the Guard ship reaching Earth.
Exactly. In fact the logical thing to do (if you can't quickly use a MatTrans system, and while I don't buy the size being impossible, I'll buy that building one capable of reaching the Bia system from Earth would take more time than they can spare), is to send two ships forward in advance -- [an] Asgerd ship for any fighting that needs to be done and a Transport ship for the first rounds of evacuations. It takes Dahak 9 months to make the trip, the Guard ships could have reached Earth, wiped out the scouting force, filled the transport ship with refugee's and as you say be returning for it's second load about the time that Dahak and the unmanned ships arrive.
(Also, there's a bit of a left over plot that is never addressed, Colin says that the "nearest" Battle Fleet units are over 800 lightyears away (by which I assume he means less than 900 lys). Those ships should arrive via hyper before Colin leaves the system.
The "nearest" Battle Fleet units could, indeed, have arrived in the system before the Guard departed, but they could not have been rendered combat ready. I suppose I should have dealt with that and, in fact, it probably would have made sense to bring them along to Earth if Earth had the yard capacity to make good their defects once they arrived in the Sol System. In fact, it did not, but the Fabricators might have been able to do the job (assuming they didn't have anything else they needed to be doing). Instead, Colin had them return to Birhat and wait there until Tsien arrived, where they became the first wave of the ships he was reactivating as part of the Insurance Policy.
Uhm, you mean where on page 179 of The Armageddon Inheritance Chernikov says "I said, sir, that we have here both an Enchanach and a hyper drive, engineered down to a size that fit both into a single hull. I am not yet positive, but I would judge that the combined mass of both units is less than that of Dahak's Enachanach Drive, alone." (emphasis mine).
Also, given the strategic advantage of having both, even rebuilding should have been considered -- so it isn't on the table for very long ("20 years!!!! Too bad.").
I suspect I'm not going to change your mind (after all, what do I know -- I'm only the author
!), but I think you're missing my point about the drives. In many respects, these ships are built -- one might almost say wrapped -- around their drives. An Enchanach drive, in particular, runs throughout the entire hull structure, with generating ("drive nodes") elements placed in precise geometric relationship to the center of mass. While the Fourth Empire had managed to engineer the drives down to a size which Vlad estimated was less than Dahak's Enchanach drive alone. This is still an extremely large unit which has to somehow be installed at the center of a 3,000+-kilometer ship. The engineering required would dwarf the PDCs being built back on Earth and take considerably longer than the estimate Mother gave Colin for reactivating the essentially undamaged units of the Guard. In the long run, it would have been far more cost-effective to simply build a new planetoid with later technology and shift his computer core into it, but the time available made that impossible. Had the original Dahak survived, they probably would have done that; as it happened, his subsequent success in solving the differences between his own computer "body" and the Empire's later technology made [that] unnecessary because hs bootstrapped himself into Two.