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 have backtracked from the End to find a coherent explanation of the duality plot lines of the Island and Sideways realms.
As the previous posts have concluded, the pivot point character was Desmond. He was the character that began to bridge the two realms and bring the other Lost Souls to the church. But it took him a great deal of time and events for him to come to the realization that he was dead and further, accepting his death in order to reconnect with his past in order to move on.
The critical points in the End can be summarized as follows:
At the Sideways benefit concert, Eloise Widmore joins Desmond, saying that she thought she made it clear that Desmond should stop what he's been doing. Desmond says she did, but that he ignored her. She asks, "And once they know, what then?" and Desmond answers, "Then, we're leaving." With concern she asks if they are going to take her son. Desmond assures her, "Not with me, no."
Jack arrives at the concert after it has ended. Kate is there and he says he is looking for his son. He recognizes Kate and asks where he remembers her from. She tells him that she stole his pen on Oceanic 815. Jack is confused, he says "and that's how I know you?" Kate says that is not how he knows her. She goes up to him and takes his face in her hands telling him how much she has missed him. Jack flashes, seeing images of himself and Kate on the Island, but still resists. She tells him that if he comes with her he will understand.
She takes him to the church, the place Jack was going to have his father’s funeral. She says they are waiting for him “once he is ready.” Jack asks, “ready for what?” Kate responds, “to leave.”
At the reunion with his dead father, Jack comes to the realization that he is dead. Everyone is dead. Christian explains to Jack that they aren't leaving; they're moving on. Jack asks where to, and his father tells him, "Let's go find out."
If one objectively looks at the dynamic of the End as the solution of Six Seasons of LOST mysteries, only one key plot point was revealed and resolved: Eloise's demands to Desmond to stop what he was doing so Daniel would not be taken away from her.
Eloise Hawking Widmore is the Key to LOST.
Eloise Hawking is the only character that had "full knowledge" of the Island events and the Sideways world consequences. It is quite the simple but straight forward explanation lacking in the convoluted plot twists of the series that everything could truly be explained through the actions, manipulations and motivations of Eloise. The only issue truly resolved in the End was that Eloise did not "lose" her son, Daniel, to the 815ers who were "moving on" into the light at the conclusion of church meeting.
Now some may complain that the show was really all about Jack. But did Jack finally resolve his father issues in the End? No. Did other characters resolve their issues? Sayid and Nadia: the opposite happened. Locke, who was abused in the island world, left without anyone. Michael did not get over his issues with Walt, so he is apparently left with the guilt of an island ghost. The island events were themed by significant "daddy issue" subplots. But in the mirror realm of the sideways world, there is only one clear "mommy issue" plot: Eloise and Daniel.
The simple bridge between the Island and the Sideways world was a simple, personal, introspective "awakening." In the End, the only thing the 815ers did was realize that they were dead and accepting their deaths. Knowing that someone would attempt to "awaken" Daniel (most likely Desmond since Daniel considered him his metaphysical "constant"), Eloise attempted to keep that knowledge from all the characters in the Island realm.
As previously posted, the story premise has to be the Island and Sideways worlds are spiritual planes of existence. After death, one has a reincarnated new life. And in this new life, you may not realize that your human existence is gone because everything seems so "real." Using the concepts of the split between the ba and ka from ancient Egyptian mythology, Eloise could craft or control two separate spiritual planes to stop the characters' ba and ka from reuniting ("awakening") and moving on to a different plane of existence in the after life.
Eloise's role in both realms could be considered the puppet master, a gatekeeper of souls, the wizard behind the curtain, the high priestess of death. Only she knew knew the awakening rule. She confused the lost souls into believing that they were still alive. They were told that their survival depended upon running through a maze of dangerous missions. The Island was really a place that Eloise created to contain any person who could lead Daniel to his awakening in the Sideways realm. Eloise was motivated by the fear that she would lose her son forever.
Eloise was close to Brother Campbell. She used that relationship to get Desmond into a monastery so he could be locked away from Penny, a connection to Daniel. But that plan was ruined when Penny came to pick up wine at the same time Desmond was being kicked out for ill behavior. Eloise used Widmore to continually throw a wrench into Desmond's relationship with Penny. She used Libby to get Desmond the boat that he thought he could use to win a sailing race to "prove" to Widmore he was worthy of Penny.
After Sarah's surgery, and after promising her a "miracle," Jack takes a jog in a stadium. He notices Desmond who is jogging up the staircase next to him. Jack races to catch up to him but rolls his ankle and falls. Desmond comes to Jack's aid. After Desmond asks why Jack was "running like the devil was chasing him," he discusses Sarah's procedure and how he made a promise he couldn't keep. Desmond wonders out loud whether he actually did save her, but Jack tells him it would be a miracle. Desmond also leaves with the foreshadowing quip, "See you in another life."
Desmond takes Libby's boat, but then becomes shipwrecked on the Island. He is put to work pressing the Numbers in order "to save the world." Eloise tells him that his fate is to press the Numbers. That his sole destiny is to stay in the Hatch and forget any notion of being with Penny. For three years, Desmond accepts that role. It is only after he begins to be aware that he is "being conned" by Kelvin, that he fails in pressing the buttons. In that moment, he is aware that he can leave the island and return to Penny. But what stops him? The sudden arrival of new characters, the Others and the 815 survivors, who again, occupy Desmond with obstacles on leaving the Island (and returning to Penny).
It is an elaborate "con" that Eloise attempts to maintain; the only problem is that Desmond and the other characters retain their personalities and "free will" to make choices. The critical choice for Desmond was to use the fail safe key to "die." Even with that sacrifice in his mind, Desmond really did not want to "die," but get back to Penny. The island realm does not "kill" spirits such as Desmond. They continue to live until they accept their death in the real world.
The events then lead Desmond to Penny in the Island world. In order to stop the bridge from forming, Desmond is still "pursued" by Widmore in an alleged attempt to keep Desmond and Penny a part. But Eloise knows where Dez is. She allows him the fantasy of a life with Penny and his "son" in order for him to keep the illusion of living forefront in his mind. By doing so, she risks flashes of the other realm and concepts of death to overwhelm Desmond so that he could awaken and spoil the critical balance Eloise is trying to maintain in the Sideways world.
In the Sideways world, Eloise is queen of the land. She has a doting son, totally under her control. She has got the wealth, status and privilege of society. Charles Widmore is a corralled provider of all her needs, including keeping SW Desmond in check as Widmore's respected, trusted number two man (which is the mirror opposite of Island Desmond whose focus was trying to get the respect and trust of evil Widmore.) She has to keep Penny away from Desmond in the Sideways world. It is too dangerous if they have a relationship in both planes. That is why Widmore has Desmond putting out corporate fires all over the world. It is unintended consequences that lead to Desmond to be in LA at the same time Charlie meets Daniel for the benefit concert.
The Sideways and Island realms are mirror images of each other: they are not exactly the same, but mere reflections upon each other. But if one holds up a right hand looking in the mirror, it appears that the reflection is holding up its left hand. If one looks to try to formulate this distinction in fundamentals between the worlds, this sums up each existence:
Island world: live together or die alone.
Sideways world: die together or live alone.
Since there had to a balance between the two partial realms, one could write a formula:
Sideways = Island
DT/LA = LT/DA
Eloise did not want to live alone without her son, Daniel.
That is the key motivational force weaved throughout the End.
So Eloise knew what the final formula was in order to manipulate its components.
Using math to move the components, a new formula emerges:
T = DT x DA/LA
T = 2D/L
"Live Together = 2 Die Together" (LT = 2DT)
Here may be the underlying principle: One needs to die twice in order to live together.
This was the one clear Truth from the End.
Eloise "lost" her one chance to be close to Daniel in the Island plane or in their real life; and she knew she would lose him if he awakened in the Sideways plane. That is why she attempted to imprison Desmond in the Island realm, along with any other person he had contact with, in order to stop their spirits or knowledge from crossing over into her fantasy life in the Sideways existence.
So one could conclude that the core of LOST was one twisted, manipulative, selfish mother (Eloise) attempting to keep her son (Daniel) with her forever.