This theory makes a lot of sense to me. Especially the parts about extra moons, taming the seas and world-building.
I have always wondered why the World Nobles are also called "Celestial Dragons". I mean, I get it could just be an exaggerated title but if their ancestors really did win a war between different moons then it would be a literal title.
I have also wondered why the Ancient Alliance has been referred to as the "creators" of the world. If the Ancient Alliance did indeed divide all the islands, move continents and destroy moons, then it would make sense why they would literally be the creators of the world. It would also explain why the planet is conveniently divided by the Red line and Grand line running perpendicular to each other with Mary Geoise at one of the intersections.
Also it would make sense that 7 moons would make for untamable seas and I think it would be excellent motivation to get rid of those moons. If all the islands were indeed one pangaea then when those moons aligned over the pangaea they would flood it, creating the need for a ship like Noah or a Fishman island.
Also when people talk about the ancient weapons, Pluton obviously has to with death and Neptune the seas, as their respective deities do, but Uranus typically is associated with the Sky or the "Heavens". Maybe there is something to Uranus moving the moons.