For the past week, I've been reading the theories about island memories "bleeding" over to the ALT timeline. My theory is based on the same concept as I think it fits the LOST timeline very well...
In my opinion, LOST produces have taken the concept of "You bleed when you get hurt" literally as we've seen that when some of the characters get hurt (i.e. Sun getting hit on the head on the island, Jack's mirror scene relating to his operation...) they start to notice something in the mirror in ALT timeline. Sure this doesn't fit with all the characters because there may be something more to it.
So taking this concept of characters getting hurt and bleeding their memories into ALT timeline, we may see that when the characters receive ultimate hurt, death, their soul will completely transfer over to ALT bodies with or without their island memories. We saw this when Juliet was about to die and realized that their plan had "worked". She was possibly switching back and forth between the two timelines. For this reason, Juliet may possibly be the only person right now to know about the island in the ALT timeline.
This also fits in the MIB wanting to kill the candidates theory because he simply wants to kill them, so they are permanently sent to the ALT. What we may end up seeing in LOST finale will be all but one characters (the candidate) dying and getting transferred over to the ALT. Jacob may be trying to kill the candidates in their ALT timeline, so they can be preserved over to island timeline.
I think this theory works really good with some of the things we've been observing in season 6.
I would really like to hear what you think of this theory some holes that I might've overlooked.
Thanks guys.