Well, here it is, Season 6, and now we have alternate timelines or parallel universes as a very real construct of the show.
Quantum physics and science fiction both provide many consistencies with the things that are happening on the island and off.
There is a great sci-fi story called 'Cascade Point' by Timothy Zahn that has a character who is able to perceive how his life will diverge down infinite branches of his future, based on his personal choices in the present. The cover of my edition shows an illustration one of those infinite reflecting mirrors that you look into and see yourself dwindling out into space.
And if you really want to hurt your brain, read “The Best of All Possible Worlds: Mathematics and Destiny” by Ivar Ekeland. It has pendulums and stuff in it.
Also read the wiki entry on the Many Worlds Interpretation and Quantum suicide thought experiment at http://en.wikipedia.org/wiki/Quantum_immortality.
The main characters in Lost seem to have memories that bleed through between each alternate viewpoint, so that's why they sometimes remember people and situations. This is because they crashed or visited (were brought there) the quantum anomaly that is the island, which acts as a window and a universe splitter between these alternate worlds.
Even if every free will decision they make causes them to travel down a branch of one particular future, their genetics and environment make it likely that each one of them will have the same or similar fates in most of those futures. Whether Kate eats an apple or smokes a cigarette today, if she's pre-disposed to cancer, in a lot of her alternate futures, she's gonna die of cancer. And RIP Charlie.
Having said that, there ARE major things the lost characters could do that would completely change the branch they’re traveling down... Like a nuclear bomb. Or MIB getting off the island and going 'home,' presumably to wreak havoc. And if they can see some of the possible outcomes, they could change things so they end up on a branch that has things going well, not bad.
These Losties may be subconsciously looking for their personal 'best of all possible worlds.' I am sure that Jack is trying to figure out a way to be with Kate and not be a drunk. Desmond will always be a loser, unless he unites with his constant, Penny. And imagine what Ben would do to be on the branch where Alex doesn’t die in front of him.
Unfortunately, I have a bad feeling that the S06 finale could be a giant pick your ending like that cheesy DVD of the movie Clue. I have thought this ever since I did not believe Darlton’s statement that ‘we filmed alternate people in the casket’ because we wanted to spoil the spoilers. BS!!! I have always believed that casket business was part of the end game.
See my other posts on the suspicious continuity errors and the alternate people in the casket theory.