Irony,
This is what I've heard regarding the show:
1) Yes, there is going to be a season 3. I've heard that it's set to start in October. I believe that I've heard October 4.
2) I've heard that the series, as they have it planned, is supposed to go for somewhere between 5 and 7 seasons. That's assuming that the network doesn't decide that it's too great a cash cow to let go, and extend it longer than they should.
3) As for this one, you're making some assumptions. You really have to be careful about that with this show, that's what they rely on the viewer to do in order to distract them from what's really going on. We don't know that the Others are the good people. We only have Henry's word to go on in that regard. And, good/bad is really a matter of perspective, and point of view.
The others did kidnap a lot of the survivors, but remember that Goodwin told Ana Lucia that they are "in a better place" now. They may be doing them a favor, or at least think they are. And they are killing people that they say are not "good" people.
We don't know what the Others want with Jack, Kate and Sawyer. Hopefully we'll find out next season, but the way this show is, we may not even see them again until season 4, or at least until way late in season 3 the way they did with Walt.
As for Henry being the leader, well, that's another assumption, though it could turn out to be true. He definately seems to be pretty high up on the totem poll, but we don't know that there isn't someone above him in the Other chain of command. And whether he was caught by accident, or allowed himself to be caught remains to be seen.