dude honestly, my take on walt is this:
it wasnt walt. nor was it walt all the times we saw him shushing shannon (remember from like season 2?). its just the metaphorical 'magic box' ben told locke about. the same 'box' that let jack see his dad, let kate see her horse, etc
walts with his dad, maybe theyre dead, maybe theyre not, but i definitely dont think hes on the island. also hahahahaha puberty owned him