Uh, because the bolded part right there is headcanon. Just because your interpretation isn't contradicted by lore doesn't mean it's accurate. Your entire post read like fanfic. I mean, it was good fanfic, but still fanfic.
It's an interpretation, I don't claim it as fact nor canon, and I wouldn't use it to make an argument. Legion/Geth VI engages in more than enough propaganda and deception to garner my distrust without confirmation of those extrapolations.
You asked me a question as to what I thought about your assertion to Legion's sentience, which is equally if not moreso a "headcanon" conclusion when compared to the inferences that I elucidated (moreso because practically all in universe expert authorities on artificial intelligence seem to be the ones whom most strongly disagree with the supposed sentience of those AI's).
Then this should have been in the game. I generally don't read video game stories, but this really should have been in the game. This is the biggest problem I have with Mass Effect 3. So many plot points are just dropped and shoved in other things or just disregarded altogether. Considering how many plot points were dropped and how snippy the narrative is in the games at moments, can you blame me for not wanting to subject myself to reading it?
The issue here is that you die regardless. I literally said I kind of thought everyone was being stupid in ME3. I said that in one of my first posts. It's not just the Quarians here, it's literally everyone. That's why I found ME3's plot to be so insufferable TBH. The whole thing feels like the entire galaxy receives some of the worst tunnel vision know to fictional media that I've ever seen.
Agreed, you should read it along with Revelations if you are interested in the setting. The latter paints a very different picture of the geth-quarian conflict that is closer to Terminator than Commander Data, and I frequently have to cite it when people take ME3's geth propaganda as gospel. Both novels are much better than ME3 narratively. Deception is terrible though, and good thing too because it is essentially non-canon.
Some of them aren't really being stupid.
The humans and turians are as stupid as any other Council species before the conflict, but start doing work few questions asked when they can no longer deny the threat. The humans do lose points for the "WE MUST DEFEND EARF" crap, though, considering it doesn't have much more strategic importance than any other Council world up until the Citadel is moved there.
If the krogan don't get their genophage cure the galaxy continues to ostracize them and they die out in a couple generations, or the Reapers wipe them out in a war that lasts a couple of generations. Their actions are extremely selfish, but their motivations are easy to understand. They are screwed either way without a cure, and have no reason to care if everyone else goes down with them or survives because of them if they choose to help or not help unconditionally. They already saved the galaxy once, after all, and didn't even get a Council seat out of it. Their concerns are validated in game by the Dalatrass's own sabotage plan, which you can take.
The quarians face a similar predicament. Even assuming the extremely unlikely possibility that the Liveships survive the war (losing even one of them while all the agricultural equipment is still onboard immediately makes 1/3 of their population nonviable) and enough people to mantain genetic diversity for a viable population, what assurance do they have that things won't continue as they were, with them being ostracized, discriminated against and bombed off of planets the Council suddenly decides are nice, until they are all dead in less than a century? Once again, damned if they do, damned if they don't without their homeworld, which they are trying to retake partly to help everyone else anyway. It's not like they get it back then are like "screw you guys, have fun losing your planet to toasters then flying around in space for 300 years like we did", they immediately throw everything they have at the war effort.
The rachni do everything they can to help. It isn't really their fault that the Reapers blow up their shiny new fleet offscreen and enslave their queen forcibly.
The salarians, asari, and geth have zero excuses, though. Sitting on the sidelines with no intention to help simply for petty ideological reasons, or actively sabotaging the war effort via tech hoarding or allying with the enemy is inexcusable.
My issue with the Quarians attacking isn't the fact that they did, it's the when. Nothing is clear in game for why this wasn't done sooner.
Except you miss my point. Even if they got their planet back, it really doesn't matter because they are going to be destroyed regardless. To put this in a more earthly sense: you have a fortress, and another war going on. Sure, you could take back your fortress, but on what grounds? What's the point? You have to leave the fortress as soon as you capture it and don't have the manpower to keep it-- in this case, the reapers WILL kill you regardless if you stay there. Your entire paragraph misses one key point: I said the timing was stupid. And it couldn't have been at a worse time. Considering how the plot of ME3 makes it out that the galaxy is being completely wrecked by the reapers, there really isn't much of an advantage Rannoch would net the team outside of "Quarian help." Even with the knowledge that Quarians would be extinct in 90 years without finding a new planet, the war with the reapers will not conceivably last 90 years at all.
The quarian offensive strategy relies on deployment of the anti-Lidar weapon developed by Rael'Zorah and later completed by Daro'Xen. There are only like 6 months to a year between ME2 and 3 (the exact amount isn't clear due to Arrival). Xen presumably needed time to complete the weapon, and the quarians to complete retrofits to arm the Civilian ships and mobilize their forces (including the recall of as many pilgrims as possible). We aren't sure they even had information of the Reaper invasion before starting the offensive, and in fact it is most likely they didn't. We get an article on the Specter terminal describing their mobilization and leaving from Illium for the Veil upon our first visit to the Citadel mere hours after the Reapers hit earth, and who in Council space has a hotline to the quarians that could get the information to them within hours (or would consider such a priority at such a time)? By the time we meet them halfway through the game, 17 days have already past in their campaign. It's likely they were attempting to complete the plan before Reaper arrival (given they had already acknowledged the existence of and committed to helping Shepard with the Reapers years ago) so their forces would be ready, but simply got the timing wrong by literally a few days to a week.
Your fortress analogy assumes that the quarians were retaking the planet to simply park and ignore the war on it. Comments by Admirals Gerrel and Raan directly contradict this, as do comments by Tali. They want to help against the Reapers, which is part of the reason for retaking the planet in the first place. I suppose you could assume they are all lying, though there isn't any precedent for that from any of them so I don't know how reasonable such a claim would be.
Resident anthropological expert Liara seems to think the war would last "at least 100 years". The one with the Protheans lasted several centuries. I'd say it's entirely concievable that the quarians could have floated around in space for 90 years until they had no working ships left to live on as the Council species continued to resist the Reapers, though they likely would have been attacked long before that. It's beside the point since the quarians never planned on doing such a thing, but they are extinct either way without Rannoch so there's little reason for them to care otherwise.