A working theory...
During Season 6 Episode 3 John Locke interviews with temp agency Hurley owns. Before Rose, the supervisor, steps in, he is questioned by the same psychic his father hired in Season 3 Episode 10. On the wall behind Locke are two posters that prominently say the word "dream."
(A scene later in the teacher's lounge says "Forget the present, plan for the future" which suggests posters aren't accidental, another one says something about "lessons teachers teach last forever.")
All of these flash sideways things seem to be more hopeful than the bleak realities before....even Locke's story was a touch more hopeful. Hurley doesn't know how Locke was paralyzed so that would explain why in the flash sideways his dad could still be invited to wedding. All the meetups are too cooincidental in sideways things - almost like terrible episode of my name is earl - and the Kate sticking around stuff in episode 2 was especially contrived.
A few years ago I thought that the show's flashbacks might be a red herring ever since Season 2 Episode 18 when Libby mysteriously showed up in same asylum that Hurley is in. But later I assumed I was wrong when the flash forwards appeared to be more honest. Maybe the sideways and backwards flashbacks are Hugo's dreams...but forwards really happened.
3 forms theory
Jacob and MIB can assume three roles each. Along with their original forms, they also can assume the bodies of anyone killed on the island, and MIB can become the smoke monster and Jacob can become the projection or spirit of children who are born or live on the island. MIB was Ben's daughter's and smoke while standing nearby as Locke during Ben's encounter under the temple. Jacob still exists in the form of Christian Shepard (perhaps only his spirit), and perhaps is stuck there, just like MIN is stuck in Locke's body.