Things you've never did in the trilogy

Any speculation as to the numbers sent and distributed is just that, speculation. It's never officially stated so we don't know. What we do know though is that Sovereign and the geth beelined it for the Citadel, they didn't stop and engage ships along the way. This is specifically stated to be the failing of the Council's strategy. They thought Saren would have to stop and fight along the way, they didn't count on the Citadel being attacked directly and for Saren to have a backdoor in. Alternatively you could read that as Sovereign smashing through the forces along the way though that wouldn't account for all or an overwhelming number of them as there are many paths to the Citadel and it only took one (meanwhile the Council spread it's fleet in all directions). It may also not have been the one that's a straight line from Virmire. Sovereign's direct but it's not stupid. A surprise attack tends to work better if you come from a direction you're not expected to.

Possibly, but this would make the choice moot altogether. If every fleet jumped back in along with the Alliance, the latter would hardly be necessary to save the DA. They could beeline for Sovereign undisturbed while the Citadel Fleet rescued its own or head for the DA and let someone else go straight for Sovereign. Either way, both bases would be covered.

Obviously the writing here is not airtight. But your conclusion doesn't follow. You can make the case I did above, that with the relays reopening, fleets should've poured in up the ass from every direction, which would've made this choice a non-issue. I suspect they really wanted the choice though and didn't bother patching the holes (not the first or last time this would happen). The delay of those other fleets is what you have to hand wave or headcanon. Perhaps not all relays unlock simultaneously, and the Alliance simply got lucky that they were the first to be able to jump in.

The facts of the case are this: the Alliance is the first on the scene, they have a choice of stopping to help an ally or focusing on the enemy (classic paragon/renegade shenanigans). If helping the ally is a shatterpoint for the whole situation, then again the choice is moot, is it not? A choice with only one right answer is not much of a choice. By that same token if I turned around and claimed that without the Alliance fleet at least distracting Sovereign by forcing it to divide its attention by slaughtering them, it would finish opening the relay before reinforcements arrived, that would again throw the choice part out the window. So the intent is that ultimately both choices are viable. So we have to work out the details to make it so since apparently they didn't.

Then there's also the issue of equating the DA with the fleet as a whole. The claim that without the DA the fleet is useless or conversely that saving the DA will provide you with all the rearguard you need. Nonsense. The DA was overwhelmed, its Guardian lasers and barriers offline. They were also evacuating the Council. What kind of rearguard would they provide? Shields recharge and weapons can be brought back online but if the claim is that the geth speed after the Alliance, there's not enough time for that. And again, the Council's on board. Not only are they shown to be selfish and shortsighted throughout the trilogy, but from an objective protection point of view, trying to guard multiple separate targets is a fool's game. If the DA is charged with keeping the Council safe, keeping the Alliance fleet safe is a contrary objective. Strongest ship though it may be it's still the Citadel version of Air Force One at this point. Charging back in to provide cover for the Alliance would be counter to their purpose. No, in all likelihood once you save the DA it retreats to a safe distance and probably get an escort so it doesn't get overwhelmed again. Which, ironically subtracts from the number of Citadel Fleet ships available to act as rearguard.

Again focusing on Sovereign is the right call, by all accounts, for reasons I've already stated in addition to what I just argued. Saving the DA is the idealistic "no man left behind" choice but it's not the pragmatic choice, nor is it the only choice by any means. And as other people have been quick to point out to me, Shepard is a groundpounder, not an Admiral and he's the one that makes the choice. Hackett is the admiral, it's his fleet and he had the readouts. If Shepard's order was tactically non-viable he should and would have countermanded it. He didn't, therefore it's viable.

Also final btw, the Alliance has such a big role in the fleets in ME2, not just because of the casualties but because with the DA gone, the asari pretty much pull out of their obligations and leave their share to the turians. Talk about sore losers.