To answer some of your questions:
3) You win it by looking for something very rare(I think, it's the rarest search you can do) at the excavation site, and it's the key to sector 5, secotr 7's been demolished and you can't get back in.
4) YOU CAN'T REVIVE AERIS. The tissues are just there for the game's item collectors to be able to get every last item, like the 1/35 soldiers or whatever. Don't bother trying to revive Aeris, you can't.
6) I forget what the item is, but there is an old man in the town east of Midgar(The one where Clous tells his really long story). He tells you some rare items that he wants you to bring him, and one of them is a book(if i remember correctly). Go to the underwater reactor in Junon and fight battles where the battle background is a tube underwater. You will fight ghost ships, which(I think) give you the item he wants. Either way, one of the enemies there will give you the book, and when you take it back to the old man, he will give you the underwater materia.
7) I think S is the cream of the crop.

Sorry I can't be more helpful but it's been a while since I've played. Besides, giving you most of the info makes it more interesting and rewarding, right?