1. It's hard to tell, but they don't look older. So I would say directly after KH2, when they arrive at Destiny Islands and get that letter from Mickey.
2. He explains that he was angry at SRK and wanted revenge and by helping/help forming Organization XIII, he'd help his past-team to regain what they lost. I don't know what they did (havent played BBS) but he forgives them for what they've done, apparently, and wants to help them.
That coat is just a reflection of the fact that he was ultimately the leader of Organization XIII, as in, the mind behind it. Or at least, that's the message I got. Xemnas of course was No1, but he didn't create the organizaiton.
3. Well, right before that he says he wanted to help the people that were victim of the experiments at Radiant Garden (Lea, etc.) and I guess what that quote means is that they're basically sleeping at this stage and need to be born again. Or something like that.