I think that Charlie probably didn't really have to die, that he was so hung up on Desmond's visions that at that point he sort of gave up. I'm guessing the door couldn't be locked from the outside, maybe, so perhaps he figured there was no way to seal off the room if he left, but I'm not sure if that was the case, and anyway you'd think they'd have time to put on the gear before the whole station flooded.
And Desmond is putting too much stock in these visions of his; at first, he was using them to save Charlie's life, but then he was letting them guide him into leading Charlie into danger. The first time it resulted in them finding Naomi, who evidently was bad news, and meeting up with Mikhail, who told Ben what the castaways were up to, which was definitely bad news. The second time, it led to them getting in contact with that freighter, which seems to be a set-up for disaster, and it got Charlie killed. I feel like somebody is somehow controlling these visions and yanking Desmond around like a marionette. He needs to be a little more careful.
I think Hurley has been very haunted by Charlie's death, and what he saw was just replaying what Desmond had described to him. I would say that the figure he saw at that point was not Charlie but Mikhail, that he was just putting himself in Charlie's shoes, but I'm not sure. Charlie's visit to him seemed very much to me like Obi-Wan coming to see Luke. I think he's definitely dead but still able to communicate with the living, though communicating is one thing and slapping somebody in the face is another. He seemed to be too corporeal to be a ghost; more like an angel, like Jonathan on "Highway to Heaven" or something.
Seems to me that all of the Others, and Ben most of all, have major problems when it comes to social interaction. How difficult would it have been to just greet the survivors of the crash right when it happened and ask Jack to fix Ben's spine and warn them that pregnant women are endangered and all that? Why couldn't they all have cooperated with each other? Seems to me that the castaways are going to wind up on the same team when they discover the newbies' sinister purposes; they could have saved one another a whole heck of a lot of heartache if they'd played nice to begin with.
It would be really weird for Walt not to have gone to his father's funeral. And Michael's mother was still around too - though not in LA, I don't think... I suspect we'll be seeing both Michael and Walt soon. I don't know how they're going to handle Walt though, when he's getting so much older than he should be. Seems like he's a time traveler like Desmond or something.
I think there are definitely people who stay on the island, probably by their own choice; I'm thinking maybe they made a pact that Jack, Hurley, Kate and the other three would tell the world that everyone else was dead, so they wouldn't go looking for them, and maybe they were able to jam up communications and hide the island again, because it shouldn't have been that hard for Jack to get back otherwise. His going back doesn't seem to be for particularly noble purposes; I think he just wants to go back for himself because he realizes he was more fulfilled there than he's ever felt back in the "real world". But I think it's like "Brigadoon," and by leaving he had to accept that he wouldn't ever be able to return. But I'm guessing he'll make it back anyway...