THE SIDEWAYS

One explanation for the Season 6 sideways plot exit into a confusing after life realm is that two points have to be taken as true.

Christian told Jack in the finale that in the sideways world he was dead (they were all dead) and that his friends died before and long after him.

In other words, people take Christian's comments as the basis that the plane crash and island events were real - - - that Jack and his friends survived the plane crash and were alive until they lived out the remaining time on Earth. The sideways universe was a "purgatory," a place where souls put themselves in limbo until their friends died on Earth.
 
Each person in this “purgatory universe” created a reality for themselves based on their lingering issues in life – that which they could not “let go” of. For Jack it was Daddy issues; Kate, the guilt of murder; Sawyer, the quest to find “Sawyer” and be a better man; Sayid, the unrequited love of Nadia; Charlie, looking for something “real” like a family instead of trivial past fame as a rock star.


The explanation is that the main characters made a connection amongst themselves because they were all  still attached to their  personal life concerns. Because of this strong island survival connection,   they never forgot the journey and growth they had experienced because of the Island, and then each character could finally understand the connections and “purpose” brought into their damaged lives by being there. With that greater understanding of themselves, they were each ready to “leave” or “move on” to the next phase of existence – i.e., the true after life. 

This view does not answer all the lingering questions, like how a dead soul can create a memory blocked sideways world with thousands of strangers based upon the "unforgettable" island experiences. Who or what directed each character's dead soul into a cosmic holding place - - - to actually live out totally different lives than the past, including Jack having a son!, and not making those "new" experiences count for anything except filler?

As explanations for the ending goes, this falls into the supernatural bin.

But look at a few other key elements of the sideways story arc. Eloise, who had present knowledge in both the island world and the sideways after life, tried to keep Desmond from "remembering" his past because she feared Desmond would awaken Daniel, who would leave the sideways world and Eloise. Why was Eloise so hellbent on keeping Daniel in the dark about his life and his current death status? Simple: she feared that Daniel would leave with the other Losties to the next phase of the after life.

But was Eloise smarter than the rest? The puppet master of the entire series? If we look at the Season 6 back story of the island, the answer would be no. Eloise would have just been another "candidate" brought to the island by Jacob. It would then seem that Jacob was the supernatural being capable of creating an after life purgatory.

Jacob did grant Alpert "immortality" on the island. And if one looks at the canon that only the island guardian can bring people to the island, then Jacob manipulated the lives of all the characters to come to his island world. There is something "mad scientist" about Jacob collecting human beings with personal issues and faults to conduct some crude and cruel experiments on them. It is then probable that Jacob orchestrated his candidates to get on the same plane in Sydney in order to crash it on his island. In normal circumstances, everyone on the plane would have died. But Jacob intervened magically and "saved" his candidates from certain death. (Or perhaps, reincarnated them as human beings without their souls - - - which were dispatched to the sideways purgatory to continue to live a false life).  The split soul concept is found in ancient Egyptian culture, which was featured heavily in Season 6. 

It comes down to a personal definition of "life." Is life the mortal existence on Earth? Or is there a life "ever after?" 

All cultures have some sort of "creator" mythology. A super being, usually from the stars,  that created all life on Earth. The same could be true for the sideways world - - - some creator imagined a continuation of the characters' airplane journey without the detour to the island - - - while Jacob kidnapped the characters to his island. Perhaps, this is the real conflict between Jacob and MIB. Jacob was the one how kidnaps souls for the island world, while MIB is trying to find a "loophole" in Jacob's game. It is possible that MIB's loophole was the creation of the sideways world (since as a smoke monster, a supernatural being that can manipulate matter and memories of human beings to take any form). 

If MIB created the sideways world in order for the characters' souls "to remember" the island, that could be the check mate move - - -  no one before had ever "remembered" the island after dying. Once the island was recalled in the after life, Jacob's game was revealed to his superiors as an unsanctioned interference with normal people's lives. Thus, knowledge of the island was the key to stop Jacob's candidate games (like a Hunger Games for lost souls).