Great Theory Chlodwig Shillingsfürst. I've always been looking for a way to add more evidence to Ramon Zaemon's walk-through in the "SH Voyage Length" thread, and this clearly demonstrates that the Moon cycle, and the time references that Oda gives like (3 days later, sleeping for 3 days, etc) aren't just random references in the script.
I think many forget that Oda has already planned out the end of One Piece, to me, this is incredible, b/c it means he's aiming towards a goal, and i'm sure he's planned out the rough storyline and storyboards for each arc till the end. Your "Around the world in 80 days" theory is another great theory, and so far, we've seen Oda pull many of our family fairlytales into his story, so what's one more.
As for why none of the Emperors have made it to Raftel just yet, it's probably due to something they're missing and are waiting in the New World to happen. We know Roger could read / write poneglyphs, and so far this is one of the biggest factors in the story involving the Void Century and the fall of the ancient civilization. This could mean there's more info contained in the poneglyphs to come in the new world, which may contain revelations about Raftel.
There's also the possibility of a submerged island, that is accessible only every X number of years.
@FFAce
You yourself don't want to believe in the 2 yrs have passed theory. Thinking about it, the Aqua Laguna is currently the only anomaly in the theory that holds any water, and I would be concerned myself if not for the fact that the 1st thing we learn about the GL is that you can't trust the seas b/c of all the mineral embedded in all the islands. We've so far been told that there are 4 types of islands (spring, summer, fall, winter), and each island has the 4 climates giving a potential 16 climatic conditions for any island, with some islands even unexplored.
I don't have time right now to analyze Clover's planet diagram, but maybe it's possible that due to the uncertainty of the seas in the grandline, there needs to be a specific requirement / alignment of the "moons" for aqua laguna to occur, and quite possibly as the arrangement of the alignment changes, the effects of the AL change, but still occurs annually.
Anyways, that's my $0.02 on this.