Status of the Grand Alliance as of the start of In Death Ground
Dan, Dan! Calm thyself.
The Alliance is undermentioned in Crusade because it never reached the degree of integration which is busily being reached in IDG. Yes, there were allied fleets with units from multiple sides, but please note the degree of hostility which remains between, for example, certain Tabbies and Humans in IDG. By Insurrection, the various navies have gone their own way again but recall that the Corporate World ploy to gerrymander the Fringe was built on amalgamating the Federation and the Khanate. And also note that the Fringe resistance to the idea wasn't racial xenophobia; it was an accurate appreciation of the plan to dilute their voting strength through another "reapportionment."
The Khanate sat out the Terran civil war for many reasons, not least the fact that if it had come in on the Rump's side, it might well have inspired even worlds which had remained loyal to the central government to go over to the rebels because of "outside" interference. It would also have violated some serious Orion honor concepts, but that's another matter.
The OADC did play a role in Crusade. By taking over border responsibilities for the Feds, it allowed them to redeploy their own units towards the war front. Remember the special circumstances of the war against the Thebans, however. The Terrans had an honor obligation to deal with the Thebans, and whether or not allies could be used was very much an open question in a war which would bring the Orions in against them if they blew it.
The Thebans do not play a role as a navy in IDG because they have no Navy. Remember? The Peace Treaty deprived them of one, and the Church of Holy Mother Terra, although losing ground, is still in control of the home world. And the Church is just likely (note that I ain't telling) to regard the Bugs as a holy retribution directed against those heretics who live on Holy Mother Terra.
As for the Bugs not having any characters one can identify with, well, let's just say that Steve and I did that on purpose. We have our reasons, which I don't want to go into at any length at this point. At the same time, however, I would suggest that "a force of nature" is precisely what they are to the Alliance, and the Allies have so far found no way whatsoever to communicate with them. In fact, they don't even know if communication is possible or that Bugs are truly "individuals" in the sense of any other known race at all.
I'm sure I haven't addressed all of your concerns, but it's about 3:00 a.m., and I have more packing to do (later) in the morning.