JOIN ME! I just stared a 5th ME1 run on a new XBL account last week, so I'll try to answer some of your questions. My main character build was the default soldier Shep in 1, mostly paragon with some renegade, and exhaustive playthroughs where I did every possible sidequest and conversation. I've since done a renegade Femshep and a neutral male shep.
1. The thing to keep in mind is that Paragon/Renegade is much more nuanced than Good/Bad. It's more Democratic/Autocratic, or Discussion/Shoot First Ask Questions later. If you max out Intimidate in 1, your Renegade options will still result in those optimal outcomes, it's just that people may not agree with your 'ends justify the means' attitude. You can be a total prick to everyone in the galaxy, be completely neutral with your squad, and still have at least two people wanting to bed you before heading to Ilos.
2. Key sidequests would be all the ones involving Cerberus, Rogue VI, and I would argue the Geth Incursion. Just finished that one this morning and the end script mentions a vid of a Quarian singing a song about 'lost planets and innocence'. Left me wondering if it was a foreshadowing of Legion's revelations about the Schism. Also, Helena Blake, Conrad Verner, Gianna Parisini, the one with the Turian general and Asari consort. All those characters come back in either 2 or 3. But keep in mind your character level at the end of 1 influences how much bonus credits/materials you get at the start of 2.
3. The simple answer is 'Yes'. I went Liara - Miranda - Liara. I 'reunited' with Liara in Shadowbroker, and actually told both of them I wanted to be with them in 3, but only had my sexytime with Liara.
4. Not sure about Kelly, you may have to break off any relationships in order to invite her up to your cabin.
5. It doesn't really matter what decisions you make regarding the council. Anderson will step down and Udina will be on the council regardless. They retcon it somewhat in the codex, but 2 years pass between 1-2 so all sorts of political machinations are possible.
6. Yes you get different bonuses depending on the achievement. Some are passive (bonus 5% XP) some are active (choose a bonus power when creating a new career). See here:
http://masseffect.wikia.com/wiki/Achievements
7. Only played up to Hardcore, so can't comment. Have seen a consensus that Insanity is best played as a critical path only, with an already levelled up character. But if you've only played as soldier, I'd recommend one of the other 'pure' classes: Engineer or Adept. If you've unlocked all the weapons achievements, you can choose sniper/assault rifle/shotgun as a bonus power at the start.
8.
http://masseffect.wikia.com/wiki/Mass_Effect_Wiki Learn it. Know it. Live it.
Seriously though, that is my bible. It shows how to min/max paragon/renegade options for every mission, all the lootables on every planet, love interests, etc.