1) THACO stands for 'To Hit Armor Class 0', which is sort of a middle of the road armor class. This is the roll necessary on a 20 sided die for your character to be able to hit a creature with that armor class. Seems that a LOT of creatures in SOA/TOB have MUCH better than a 0 armor class, however...
2) I have not finished the game with my evil party. I think it depends how you play it. In general, however, I would say the game is more challenging for an evil party. Especially if you act evil and kill a few people who you don't like. Those gank squads of Amnish soldiers and Cowled wizards aren't fun if you're not fully built up...
3) Monks are great to advance in. They probably gain the most power from advancing out of any class. Between their numerous special abilities, armor class improvements, and raw damage they can inflict with their bare hands, they are great once they get a few levels on them. Plus it's a lot of fun to watch them kick your enemies in the head...
4) You won't see Drizzt again until fairly late in Shadows of Amn.
5) I never played Icewind Dale, though I understand it is more linear than SoA.
6) I'm working through the Throne of Bhaal now. I'll let you know when I do...