Occam's Razor - In short, the easiest solution is usually the right one.
Related to the Flash Sideways: What if the way that the two alternate realities converge is as simple as the course of action that will take place over Season 6, (selecting candidates, Losties being claimed, infected, and the impending 'war') sets up a series of events that creates the LA X timeline (The island sunk, Jacob/MIB may have never existed).
I believe that Jughead was simply a red herring, which might have simply negated the energy in the Swan and sent them to 2007, as we've seen some information now, including Ben and Ethan being off of the island, and other oddities that would've been different or even impossible, had the realities 'split' in 1977.
I think that the finale will show us the real reason for the split, and that the sideways flashes will show us that the Losties begin to remember each other, so that we do not feel as if the last 5 seasons were for nothing.
Simply put, Darlton are showing us the events that will happen after this season concludes. I wouldn't be surprised if one of the last scenes is the opening scene of LA X. The castaways are still destined to be apart of each other's lives, and could possibly have a happy ending this way...