These are interesting food for thought, but not even TOA really provides us with enough day-to-day information about each civilization. To my knowledge, the only 4X game with fluff that detailed is Sword of the Stars. Still, my own personal opinions, leaving out question #1 as I intend to give it more thought:
2. Which faction would cause the most civilian and environmental damage?
The Korath are genocidal maniacs, so it's safe to say they'd cause the most civilian damage. The fact that their "Aul Incinerator" technology is apparently beyond the pale of even the Drengin suggests they'd also cause the most environmental damage.
3. Which faction's society would you rather be a member of?
The Thalan or Iconians, both of which have technology that strongly suggests a post-scarcity society. The Thalan almost certainly do, while the Iconians may be more based off of their psychology and culture. Either would seem to have an extremely high standard of living for its citizens. In terms of the campaigns, they've also fared quite well compared to many of the other non-evil races.
4. Which planetary attack method would cause the most civilian and environmental damage?
Mass drivers. While it's not an option in the game, a large enough asteroid hit would wipe out all life on a planet.
5. Which faction would be able to improve their citizen's quality of life more significantly?
The Thalan or Iconians. See above.
6. Which would be better: being a citizen or being a non-citizen?
The game suggests that all citizens are subject to a military draft, which I oppose on multiple grounds, so in my personal opinion I would tend towards being a non-citizen.
7. Which Race is the most evil, in your opinion?
The Drengin or Korath. The Korath are slightly more over the top, but the flavor suggests that being a Drengin slaveling is quite possibly a fate worse than death, so it's not clear how bad the Korath exterminating everyone really would be if that's the alternative. The Drengin also are a little more disturbing by how pragmatic they are. The Korx, however, are probably the most unlikable evil civilization - they just seem so petty.