It must be something like that - that the Alex that Tina is playing is a different Alex. Maybe they will have Amber play the young Alex.
I don't like the idea of Tina being on the show, but will watch anyway. Nothing against Tina, but it just seems so odd that she would be on Lost that it's hard to get past it.
Re Walt, that is always a problem with having kids on a show like that. Unless you are fortunate enough to find a Gary Coleman or a Webster who has a disease that won't allow them to grow anymore (and I mean that in the sense that a weaselly Hollywood guy would think that's fortunate), then I would just replace the kid with a younger actor each season.