My theory is that the key to the whole show is Desmond. We already know from past episodes that he is "special" and the rules of time travel don't really apply to Des. It doesn't apply because he doesn't really time travel but rather goes to different "realities". I think that all during season 3, while he was seeing Charlie die in different ways it wasn't because he was having visions, he was traveling through the different timelines and see what the pros and cons of each potential timeline were. I think that when Jacob told Hurley that someone was coming to the island, I believe it is Desmond. Jacob needs Des to go to the different realities and figure out a way for his endgame to play out to his advantage. But instead he uses his "gift" to find a loophole where Jacob never interfered with the Losties. In doing so, he will make to where he saves all the Losties and lets them land safely at LAX and he is some! how behind the island sinking, that is why Widmore and Hawking were so hellbent on trying to convince him that he wasn't "special" and couldn't change anything because they know that ultimately, he is the one who is truly in control of the island and he is the one who can set things back to the way the should've been.
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