Interesting idea
So we know the new season is supposed to be a totally different storyline, like back to basics but at the same time something they havent done yet.
We should remember that Sarah I do believe her name was, left Jack, on the day he kissed that other girl. He didn’t mean to and even told her. Trying to fix everything of course. She left Jack and later on was with another man, after he’d gotten back from the island. We couldn’t see the other man, at least not clearly. We have no idea who that was. It was for some reason important enough to add, to even have the man in the same shot as them. What if that man was Jack?
All the losties know their lives better than anyone. So who better to manipulate the chess board, than themselves? The pieces(Jack, Kate, Sawyer…etc…etc..) are picked at random, then introduced into an ultimatum(kill or be killed) so they manipulate the game. They intersect with their lives introducing new alternate realities in which the game would end in their favor.
Alt Jack goes to Sarah telling her to meet him at wherever and she sees him kiss that girl, the original jack comes home getting left. Later on Alt Jack meets up with Sarah again winning her back but then having to reveal the truth after the crash.
[SPOILER]
This probably happened to them all. In same way at least. Anywho….thought it might be possible. Also read a spoiler that might help give this theory some certainty. I don’t want to mention that spoiler though, but it is recent in the dark ufo website. Under the hangman portion that was recently solved…about christian being a grandfather…?
[/SPOILER]

I think this theory should be removed as it contains possible spoilers.
Doesn’t every theory?
Well I haven’t read any spoilers so I”m hoping this person didn’t reveal anything from actual footage of season six. I mean we can’t avoid the comicon stuff and cast news. But if you are revealing something that is from a spoiler site about the actual scripts, then please don’t.
Otherwise you have a few facts wrong and it seems a little too silly for this show.
If an adjacent time line begins in 1977, then there are many possible ramifications. Including the possibility that any of our characters conceived after 1977 may not even be born.
Also what you describe is not an alternate time line. I don’t even know what it is. Unless Jack from 1977 survives and HE is the second Jack that you refer to.
the only thing that i may argue against this theory with is with regards to the bunnys. I believe in a online video extra of Lost we see Dr Chang talking to the camera with a bunny behind him and then all of a sudden a bunny with the same number on it’s side appears (from the past or future). this causes Chang to go into a bit of a panic saying somthing like the 2 bunnys can’t go near oneanother for whatever reason. If this is the case then the two Jacks probably wouldnt be able to go near each other either?