The other side was the cause of the Geth's actions. The Geth had to account for the other sides stupidity. Everything the Geth do is because of the actions of the other side. They react. Why did the Geth break off all communication from the galaxy and hold them in a rather contemptible view? Not hard to do when the organics are dead set against you, even destroying you simply for existing out of the fear that you're going to turn on them on some irrational basis of... well nothing. Why are the Quarians or other races afraid that once the Geth start asking questions about their existence, it will automatically lead to a terminator style conclusion of purging organic life from the galaxy? What would make the Geth care about organics when the organics have never held them in any regard? Why do the Geth have to suffer for the idiocy of the Quarians?
AI was already outlawed by the Council at the time. This would have been a huge influence on the Quarians' course of action (we see what the Turians did at relay 314, Anderson recounts what the Council was willing to do to stop the Batarian AI research project (his mission with Saren)). Computer experts the galaxy over believed an AI would be capable of subverting the IT infrastructure on which society depended (the technological apocalypse Gavin Archer refers to). I've seen the argument that the Geth are not accountable for indiscriminately killing off the Quarians because they weren't really conscious of what they were doing (basically, that the Quarians as a species should rightfully suffer for the Geth's idiocy). The flip side of that is that the Quarians were not aware they were destroying a self-realized AI; rather, a network of VI's which were beginning to show the traits.
The loss of 99% of their population in such a short time seems evidence enough that the Quarians were right to fear what the Geth were capable of doing to them. Their failure was in their assessment of Geth motives prior to issuing the shutdown order. But can you blame them, really? How safe would someone feel knowing some entity could drop their aircar out of the sky tomorrow just to see what it looks like? Consider the example of Jarrahe Station in ME2: refraining from aggression towards an AI/rogue VI does not guarantee your safety any more than it would with a wild animal.
The Quarians were replacing much of their military hardware (and front-line troops) with Geth platforms ("created as tools of labor and war"). Honestly, what do you think we'd do if the computers in charge of our nuclear arsenal started deviating from their programming in any way?
Nothing good can come from reducing solar output. The only thing we don't know is how much they ultimately would have blocked off. To repeat what I said on page one, I can understand why the Geth were wary of letting the Quarians near their dyson sphere. This isn't an unreasonable concern - self-interest would dictate that they deny the Quarians access to Rannoch in order to protect the sphere, but this in turn (particularly when the Reapers came knocking) puts the Quarians' survival at risk and increases their desperation, thus the likelihood of conflict.
Really, the Geth couldn't have picked a stupider place to build it. Even without the Reapers lighting a fire under the Quarians' collective asses, what did they expect? Unless the Quarians use (illegal) genetic self-modification to eliminate their dependency on their native plant life, they
will have to come back if they're going to survive. Because of the Geth's own isolationism, even in the face of organic attempts at outreach, the only option the Quarians are left with is to reclaim it by force because the Geth have given no indication whatsoever that they can be reasoned with. You invoke the AI ban as reason for the Geth to suspect organics, but that door swings both ways.
If they built their sphere elsewhere, Rannoch could have been returned without any risk to the sphere.
I don't know about that, since it was portrayed otherwise. How difficult would it be to actually fill their air with oxygen (and the ventilation ducts the ship has)?
There's more to life support than simply pumping in oxygen (assuming their ships are even airtight to begin with). Long story short, it'd be a major overhaul.
The same legal sanctions that apply to states and corporations, not individuals. The sanctions against a state don't go away just because the people running it go out of power.
Eh. The way I figure it, when Haestrom's sun cooks off they'll be severed from the galactic community for centuries or millenia to come anyway. Levying sanctions on them post-war when they've had no hostility with Council races just seems vindictive for its own sake.
Which brings up another bad writing point - what the hell was Hackett talking about, Gerrel "causing trouble on the Turian border?" This is never elaborated on or alluded to anywhere else. The writers couldn't be bothered to mention the genocide, the centuries of isolationism, inaction against the heretics or any of the rest of the points detracting from the Geth; they didn't mention the aid the Quarians provided to the Turians in the wake of the Taetrus Blast (as established in Cerberus Daily News), but they spontaneously made up things like this? Just seems like they were further stacking the deck.
EDIT: Holy crap, that got longer that I meant it to.