re: David - if I'm not mistaken, as soon as Locke tells Jack he has no son, we never see David again. Momma Juliett leaves him at the concert, then remembers with Sawyer. Aunty Claire goes and gives birth, has her moment with Kate and Charlie. And Pappa Jack is intrigued enough by Kate and what she's telling him that he leaves the concert without David. So there's either some pretty bad parenting going on here on the part of Jack and Juliette (and some bad babysitting on the part of Claire) or maybe everyone realized they didn't need to worry about David any more. And that's just from the fact they abandoned him at the concert. Never mind that they all headed off to the Church and went off into the great white beyond without him, leaving him to fend for himself in that "life" he was in.
Take that for what you will, but notice that Juliet leaves David at the concert with Claire. Juliette has her moment with Sawyer. Claire has her moment with Charlie and Kate. Jack starts remembering, lets Kate drive him off from the Church. Meanwhile 12 year old david, if he's even still around, I guess was left to fend for himself at the concert because all the adults left without him. So I sort of felt like Locke telling this to Jack had something to do with us never seeing David again.
Agreed. It's as if he dissippated
Just like in Coraline. Coraline thinks that her "other mother" and "other world" are real, but starts to pick up hints that things are a little ... manufactured. It finally hits her over the head when she's walking through the woods and the woods slowly go from being real and 3-dimensional to pencil sketches of trees and 2D, and then to nothing at all. An empty canvas, and she's standing there among ... nothing. That's when she realizes this "other world" is what someone just created for her. There are limits to how far the illusion can go.
DAVID was part of that illusion for Jack (and for Juliet) in the ALT universe. When Locke told him that he didn't have a son, something changed in his idea of "what reality is" and we never see David's face again. The illusion of David fell apart, and David ceased to be in Jack's ALT universe.