The following is not a theory, but more like a thesis. An analysis of the elements presented by the creators of LOST in order to establish an unknown, unexpressed or clouded explanation for the premise of the show and its disjointed parts. To unlock the mysteries of LOST, which I believe have even escaped the minds of TPTB, one needs to find the Key. In the search for the Key, we find the most probable explanation for the show mythology in ancient Egyptian death concepts, including the role of duality.
A favorite symbolic thematic feature of LOST was the mirror. A mirror gives a person a reflection of one's true self. It is not an exact duplication, but a mirror image. When we try to piece together the fabric of the sideways realm and the Island realm, it is best to view them as mirror images, similar but not quite the same.
In each, characters lived apparent long, complex "lives." Similar, but not exact. One more idyllic than the other. The show's core appears to take the concept of Egyptian death rituals, including the division of a person's soul, as the stage to set up a dual character system: one part of the old individual residing in the Island realm and one part of the old individual residing in the Sideways realm. "Residency" in a metaphysical sense because each of these worlds is not "real" from our human perspective. For the island's mysteries and story lines to make total sense, those facts are not absolute truth from our laws or science; it is a spiritual world that resembles the look and feel of the Earth bound existence.
The Sideways world confirmed that the characters were spirits. They were all dead. For the mirror image duality to be in balance, the Island existence as a reflection for the SW cross-over would mean that part of the dead souls were lost in the Island realm. Only the reunification of both spirits would be total enlightenment and the ability to move on as a new whole (into the white light) to the next plane of existence. One has to accept the premise that once a person dies, they live on in the after life, in lost pieces searching for a new whole existence.
One can lift the dense fog of debate on whether the characters lived or died by simply deducing that the show was all about spirits. For if Desmond's consciousness from the Sideways (known dead) realm was interacting with Desmond consciousness in the Island realm, this realm would have to be a mirror image in order to make the connection. For how can a dead self interact with one's prior live self? It is too confusing. It is too disjointed. It does not adequately explain the connection.
Throughout the series, I picked up on the formula that "knowledge is power." Knowledge in the sense that a character finds out and believes that they are dead is when they can begin to direct their lost soul to connect with their counterpart in the sideways realm. On the island, characters had "perceived" power to manipulate others. Ben was a prime example. But if you recall, there were many messengers who told the characters that they were "dead:" Naomi and Cooper. Even Hurley mentioned at one point that he thought they were all dead, but he repressed the realization (but he could talk to dead people). When did Jack come to his own realization? After fighting Flocke, and lying down in the jungle and letting go in the End.
So each character is represented by mirror images of one's body and one's soul. There is a barrier between the realms that can only be crossed by the enlightenment or realization of one's own demise. But the tricky part is the Island characters were thrown to the primal underworld of missions, mysteries, conflict and danger that distracted them from asking basic questions or demanding detailed answers.
There was a nexus point between the two realms. And that pivot point was shown in one character, Desmond. If we trace Desmond's character backward from the End, we do not find him mentioned in Season One. He was not a major character in the series until the fail safe key incident and his "special" flashes of alleged time traveling consciousness episodes. (They may not be actual time travel consciousness but a static connection of memories from the sideways realm bleeding over into the island spirit.)
It was Desmond whose Island and Sideways souls combined first, creating one mind over matter, that led to events which brought the other characters home, together in the Sideways Church. With that knowledge, the fearful, cowardly, runner in Desmond's Island persona vanished; it did not matter if he was put on a large EM generator, thrown down a well, or shot through the heart. He knew he had his mirror image waiting in the Sideways world.
And to fully understand and appreciate the Ending as it was written, you have to accept that Desmond's transformation was the center the prime conflict for the entire series. Yes, it appears unbelievable that the Dharma-Others purge, the rage between Jacob and MIB, or the Widmore-Linus feud were not the basis for the story engine and ultimate climax for the show. But look at the final church scene: none of those Island conflicts were relevant, material or resolved in the End. There was no moral conclusion on who was good, who was evil, who was right or who was wrong. It really can be distilled into a very simple analogy: the characters split souls were lost on the story game board (a maze) needing to find each half in order to win rebirth in the after life. For the 815 survivors, the game board was constantly changing with different challenges, villains, inconsistent sciences, an inconsistent concept of Time and manipulations. Many story arcs led directly into unexplained dead ends, just like in a maze. Which is exactly what the puppet master behind the real conflict wanted to maintain forever.
But the puppet master could not keep Desmond, the pivot point between the two realms, from reaching his soul reunion. Once that first domino fell, Desmond set in motion other actions to bring the lost souls from the Island world through to the Sideways realm. The flood of island memories to the souls in the Sideways realm would have adverse consequences for the puppet master, who tried to stop the reborn characters from disrupting the Sideways world.
So if Desmond was the pivot point, his character's realization of the duality existence broke the barrier between after life realms, who was trying to stop him and why? Who was the Wizard behind the Curtain in this Oz? Who was the Key to explain the motivations, stories and ultimate conclusion of the End?