The concept of merging the sideways world with the island world makes little sense. For if the sideways world was corporeal to the original intent and story line of Lost from Season 1, why did it turn up only in Season 6?
For some, the sideways world is the happy-happy land where our island characters can be re-born or have a better life. If the island explosion-implosion climax distorts the earth spatial time line to invert it back to 1970, with no Jacob "touch" interventions, and the characters lives were totally altered to "the way it was supposed to be," then that could be a plausible ending. From a story construction standpoint, I prefer a separation of church and state: there is no suitable purpose why the sideways world and the island world need to cross over and connect if they developed separately in time and space.
The sideways world is set in 2004. Flight 815 lands in LAX. None of the characters on the plane are aware of a mysterious island (for it is shown under water). The island world is currently set in 2007, and there are only a hand full of people left alive who know that the island exists. In the island situation, the main purpose is to keep the island secret, to avoid humans from trying to exploit (or destroy the light); basically allow it to disappear. If that is the over riding purpose in the island story line, there is no need for any action in the sideways world. If the sideways world is a different linear highway in time space than the island's time space, so its actions or non-actions should have no affect on the island events.
To have any affect between sideways and island worlds, they would have to be both on the same highway. It would mean the island car is three miles ahead of the sideways car, both speeding along the same highway. Again, the separation means no overlap. The sideways world is not chasing "history" or known events. The only way the sideways world and the island world could be together is if one is real and one is an illusion (like one sleeping kid and one loud, talking kid on the same school bus, a couple rows a part from each other. The talker could "disturb" the sleeper.) The problem is that our Lost bus would contain two of every character, who led different lives or made different choices to create contradictions (subtext: paradoxes). Slamming the two together would be like rooting for a school bus crash.
This season, the sideways Desmond has been hell bent on re-connecting the sideways characters from 815 to sideways irrelevant island world memories. How does the main characters knowledge of the island in the sideways world help or solve the Smokey problem that Jack and the other survivors now face? I grossly speculated that a possible writer angle would be that the only way to kill Flocke would be a conscious connection to the sideways world, in which, sideways Jack "kills" Locke on the operating table. How or why this would affect Flocke, I don't know. It seems too far out to be the final solution, from a story connection let alone quantum physics and sci-fi parallel universe principles.
This is why I cringe at the prospect that the final ending of the show is a merger between the time worlds. How does one take two independent people and fuse them into one body and soul? Does one die? Do both die? It creates too many new questions.