MAN IN THE MIDDLE

There is one inflection point between the island and sideways story arcs. People believe it is Desmond, but the real man in the middle is a background character.

This comes about in reviewing a minor detail in the series. The flight manifest contained the names of all the passengers who flew on Flight 815.  The manifest was part of the papers and materials kept by Sawyer in his stash shortly after the survivors started to build camp.

The earliest known time that the mid-section survivors were in possession of the manifest was day 5 - Sunday, September 26th, 2004, during the burning of the plane. It then went into Sawyer's possession for approximately eleven days before passing to Hurley on Day 16.

Hurley used the manifest in order to make a census of everyone on the beach and in the caves.  He asked everyone for his name, place of residence, and the reason he was in Australia to compile the census.  It was during this process, Hurley found that Ethan Rom was not a passenger on the plane, but an Other.

Lostpedia states that only a few people have actually seen the manifest:

Locke knew Hurley and Sawyer's real names (Hugo Reyes and James Ford) from the manifest.

Sawyer had access to the manifest since it was part of his stash.

Hurley used the manifest to create his census of survivors.

And co-pilot Frank Lapidus claimed he had memorialized the entire 815 manifest so he was able to identify Juliet as not being a passenger. How Frank would have received a copy of the manifest (which is created at the gate through boarding passes) is unknown, but a telling bit of information.

In the flash sideways Flight 815, pilot Seth Norris is listed instead of Frank. Which means some details of the original flight were the same. But the  manifest was slightly different, with some characters such as Desmond,  who were not on the plane originally, were on it in the flash sideways. Other passengers were seated differently or missing altogether.
 
After having some realization about this world, Desmond asked his limo driver to get him a copy of the manifest so he could show the other passengers something. Desmond was later seen seeking out various passengers of Flight 815 and manipulating their circumstances to trigger an "awakening"  though it is unclear to what extent he consulted the manifest to accomplish his goal.

What was the cosmic bridge between the island and the sideways plane manifests?

It could be the knowledge of Frank, who is the only character to leave the island more than twice, and the fact that Ajira 316 actually made it off the island with Desmond on it.

Lapidus is a Biblical name derived from the Latin word lapideus, meaning "made of stone."

So was alcoholic, depressed pilot Frank the foundation stone on which the series can be connected?

Frank, as a character, does not get a lot of credit for being independent, resourceful, dynamic, skilled and perceptive. He knew the evil intentions of the freighter crew from day one. He knew something was wrong in the whole disappearing Flight 815 scandal. In some respects, he was the one who had a clear understanding that people and events were being manipulated, so he rode the wave of the deceptions to find his answers. As a pragmatic person, he survived the various dangers on and off the island. And when he found the 815ers on his Ajira flight, he deadpanned that he was not getting to his scheduled destination.

Or was he?  In some respects, Frank represents an guardian angel for some of the 815ers. Through guilt or heavenly redemption, he literally "saves" several people, including Kate twice. He was the man in the middle of the action, but not swept away by it.

By Frank getting Desmond off the island had to have changed the alternative time line to place Desmond on sideways Flight 815. This is the most dramatic change in the sideways manifest, as it creates the path for Desmond to remember and reunite the island memories to their souls. So Desmond takes on the role of an underworld ferryman or priest, to get his charges through judgment and paradise.