Upgrading Dahak to a hyperdrive
[I] guess that the real question -- which the book simply skips, is how long it would take to upgrade Dahak to have a hyper drive. If that could be done within the 10 months available (4 while the repair of the single ship that you want to send back crewed was taking place, and 6 after it has left), then that's what should have been done. If not, there should have been some mention of the fact that it'd take longer than that to upgrade his drive.
The question was "skipped" because what you're suggesting isn't possible, a point I thought was dealt with when Chernikov first described the combined hyper and Enchanach system aboard the Imperial Guard planetoids. "Upgrading" Dahak would have required total rebuilding (which, I suppose, is what I did, in a way, when I blew up the original Dahak). The decision to bring home the entire Guard instead of only a couple of ships was predicated on Colin's belief that under no circumstances could they beat the scouts to Earth (true); that Earth would either hold or not hold against the Scouts; but that the entire Guard would be necessary for Earth to have a chance to survive against the main Incursion (true). It's also significant (as pointed out in the books from the Achuultani PoV) that this incursion is twice as powerful as any ever heretofore sent into the sector. This was intended to imply, among other things, that the scouting forces were also twice as strong. The defenders' projections of what they could expect to face were based on the fragmentary reports/evidence available via Dahak of the previous incursion against humanity. In short, Colin had to make a call, considered his work force (and what would happen if he reduced it), his available options, and what he could project of the threat levels involved, and made his call. Might have been right, might have been wrong, but I really don't think "stupid" is an appropriate description.