All this talk about changing the time line reminds me a lot of talk earlier this season. Only then, it was about Constants. The key to stopping the time-flashes were Constants. The O6 were the Island's Constants. Desmond can only go back to the Island if Penny comes, because she's his Constant, etc. Now we are starting to sound this way about time-travel and whether any variable will change the outcome of an equation. I get caught up in it, too! I think it's due to the writers' ability to use important themes to distract us from their plot twists.
I submit time-travel is not the ultimate revelation about the Island. The most obvious reason is, it's been revealed too early! But there are many reasons not to believe the Island is just a big time-machine. Why would a time-machine have the ability to heal people? What does the Smoke Monster have to do with time-travel? What's with all the dead people walking around? The Whispers? The Statue? I think time manipulation is, like all these other things, another aspect of this Island, which is the ultimate mystery of the show.
The nature of Lost is such that we get distracted by these details, like time-travel, especially when they are introduced in mind-bending ways. But, like the Constant, it's just another weird tree in this crazy forest. The finale, I think, will put that in perspective, before another season-ending twist.
Of course, this is Lost, where nearly anything can happen - so I could be totally wrong!