CROSS OVER

In television, the concept of a "cross over" is where a star from one show makes a guest appearance on another show, in character.

We have a similar concept in the LOST saga between the characters' island/flashback lives and the sideways presentation. There are some apparent irreconcilable differences.

For example, Jack.

In both worlds, Jack is a successful spinal surgeon.
In both worlds, Jack has had serious issues with his father.
In both worlds, Jack has gone to Australia to bring his father's body back home.

In the island world, Jack is divorced from Sarah. He is alone. He only has his mother left.
In the sideways, Jack is divorced from Juliet. He is not alone, he has his own son David.

For example, Locke.

In both worlds, Locke is paralyzed and bound to a wheelchair.

In the island world, Locke is bitter and hateful against his father for causing his injuries.
In the sideways, Locke has a relationship with his catatonic father.

In the island world, Locke has ruined every relationship he had including the one with Helen.
In the sideways, Helen is by his side as his loving spouse.

In the island world, Locke has bounced from meaningless jobs and has no career path.
In the sideways, Locke is a content substitute teacher who befriends a meek teacher, Ben.

There is such a continental divide between the character's foundational base in the sideways world as to question which realm is actually the true one.

It is more likely that the island/flashbacks are real because we saw them unfold for five seasons. The sideways world would be a collective fantasy, dream world for the island characters to deal with their anxieties, fears, stress and depression.

However, there is a possibility that the sideways world is the true foundation because the characters in that realm were basically good people living normal, good lives. And is that not the fundamental philosophy of all religions that good people can go to heaven or paradise after death? The sideways world was a place of death where the souls of the characters got together in the church.  But if their shown lives were continuations of their true past lives (which makes some common sense if their souls did not want to acknowledge their own mortality), then the island and flashbacks were the collective fantasy and dream world created by the characters to continue to experience their fantasy lives or come to terms with their own deaths.

So, which cross over is correct?

In a linear thought process, the real characters crashed on the island and the sideways realm was mere purgatory fantasy holding pen until everyone lived out their lives on Earth.

In a non-linear thought process, the real characters sideways back stories were true, but their nightmare-fantasy-escapist adventures were captured in the island stories. Just like when people go on holiday, they can create new persona's or let their guard down to live a different kind of lifestyle.

So, one can cut and paste the various character time frames as follows:

SIDEWAYS BACK STORIES = TRUTH.
Island Back Stories = Fantasy/Adventure/Dreams/Nightmares
ISLAND STORIES = TRUTH
Sideways Stories = Fantasy Continuation of Lives until acknowledgement of own deaths.

This compares to the linear approach:

ISLAND BACK STORIES = TRUTH
ISLAND STORIES = TRUTH
Sideways Back Stories = Fantasy/Adventure/Dreams/Nightmares
Sideways Stories = Irrelevant Filler until all survivors died in real life.

The former approach is much more complex, which makes it highly unlikely thought process for the show's writers who seemingly too the easy way out in throwing a sideways after life to clean up the plot.