There Have Always Been Multiple Realities on LOST by EmilyG

Back in Season 4, the Losties split into two camps based on what was (seemingly) incompatible ideas: Jack's camp believed the Freighter Folk wanted to help them, Locke's camp believed the Freighter Folk wanted to kill them. But as we saw, both camps were right: Daniel's 'group' wanted to help them and Keamey's team wanted to kill them. Both Locke and Jack were correct, from a certain point of view.

I think the same thing is going on re: WHH and free will. One does not cancel out the other: both theories are correct, from a certain point of view. Two realities will emerge from The Incident: one is the reality we've known for the last five years and the other is a new reality in which the Swan is never built. This is the only way to avoid a paradox: the reality that the Losties know can't be 'destroyed' in favor of a new reality because Reality 1 is necessary to create Reality 2; however, the opposite must be true---Reality 2 must exist at the same time as Reality 1 because Reality 1 contains within it the seeds of Reality 2's existence.

I hate to use the term 'parallel dimensions' but it seems appropriate. Why is everyone so desperate to control the Island? Because the Island is the gateway. Whoever controls the Island, has the potential to control not just time, but reality itself.
True Island initiates---the Others---have the power to 'walk between worlds' (Narnia reference?), and that's what we've been seeing over the last five years---the 'dead' aren't really dead, just Losties/Others who survived in the other reality, 'walking' to this reality. When someone says 'this is what's supposed to happen,' what they're really saying is 'this is what happened in another reality.'

To see different timelines, different realities, and have the knowledge to manipulate them to your desires---that's the kind of power that could make a person practically a god. Who are Jacob and the MIB? Maybe they're the same entity, just from two different realities, meeting together on the Island. Maybe they're the two people who initially harnessed the Island's power.

It seems pretty clear that the Losties are never 'truly' going to return to the outside world. No matter where they are, or when they are, the Island keeps pulling them back in. I think they'll eventually 'harness' the Island's reality-warping power and either serve (or replace) Jacob and the MIB.