Jin was having flashbacks
Sun was having flashforwards
I think.....the bit I didnt understand was the date on Jins grave was the date of the plane crash.
Like you, I dont think we'll ever get answers....I mean the writers cant possibly link it all together. The crash, the fake crash, the dharma stuff, the hatch, the button, the polar bear, the black smoke, the others, the under water thingy, the boat, the gas stuff....
Don't read this if you don't want the plot ruined...
In the flash-forwards, the "rescued" (Oceanic 6) are spinning the story that only 8 people survived the plane crash, with 6 of them being rescued. Aaron (Clare's baby), Kate, Jack, Hurley, Sun and Sayid being the survivors, and 2 other as-yet-un-named being those who died.
So, as far as the outside world knows, Jin died when the plane crashed.
I would like to think that the writers have a definitive ending in mind, but suddenly fucking about with viewers by mixing flash-forwards and flash-backs, suggests to me that they are making it up as they go along.