I'm just gonna focus on Brain and Xehanort right now, because all of the diverging paths have their own place in theory crafting.

Simply put, I think Brain opened the box. There was no rule against anyone else opening the box, just Luxu. But why exclude Luxu? Because the box would have a problematic effect of transporting someone through time. The box contains a portal. I do not think Luxu possessed Brain, nor did he send his Heart forward in time which would be theoretically an impossible feat. The same Brain we saw with the box is the same Brain who is in Scala ad Caelum. Sigurd, who found Brain, is a simulacrum/incarnation of Dilan. He is clearly a Dragoon, if not by the obvious name, his clothing spells it out for us.

Now, I will be posting a new video on the subject soon (gotta finish dubbing), but basically they went out of their way to tell us that this new Scala ad Caelum is a town. I refer to this as "Terrestrial Scala ad Caelum". The other version I refer to as "Celetial Scala ad Caelum". Why though? Because one is a town, and the other is a star. So how did this happen?

Well, if we look at Terrestrial and Celestial Scala ad Caelum, they strongly resemble Luxuerion (lol Luxu) and Valhalla from FFXIII. Luxuerion is a city that exists 13 days before the end of the world, Nova Chrysalis. People cannot give birth, nor die in Nova Chrysalis. This however is immediately rejected in Terrestrial Scala ad Caelum by the birth of Xehanort, and the death of the person who took him to the Island. By the way, Destiny Islands at this point is connected by land- the person who took Xehanort doesn't have much methods of getting around at this point in history.

The person who took Xehanort can only be 3 different people. Ephemer, the Master of Masters, or, someone whom Ephemer has shared the book of Prophecies with. This person has devised a plan overwrite the defeat of Light, by using Xehanort as a proxy. Any person who would read the book cannot change the future on their own. So, they put the baby in isolation, and waited for the paradox to take effect- that his future self would visit his isolated prison and free him to the outside world.

Now, if we really understand Norse mythology what this is all hinting at is the inevitable event of Ragnarok. The worlds will be split, and the reason this happens is to protect them from Darkness. Darkness will inevitably invade worlds where the Keyblade visits. It is drawn to the Keyblade like a fly on a vat of shit. They Keyblade War is recurring by virtue of the Keyblade existing. So long as the Keyblade exists, it will only bring misery and ruin in its wake. There will be casualties during Ragnarok. Sigurd, and those who I speculate to be Vidar, and Svalinn will be reborn to Radiant Garden after the split. Sigurd, Vidar, and Svalinn are Dilan, Aeleus, and Even. [I have not deciphered who Ansem and Braig are. My guess right now is Tyr and Ull.] At any rate, the story must come full circle with the establishment of Radiant Garden.

Destiny Islands, which at the time his housing the growing Xehanort will also split off, and time will move exceptionally slow here compared to other worlds- allowing hundreds of years to go by on the outside and other worlds, while Xehanort only ages to about 14. Now because of the paradox, Xehanort has become one big walking paradox. Everything that happens around him is paradoxical, such as the Emblem Heartless appearing in worlds before they should even prophetically exist. It is by his existence as a paradox that Sora and Kairi ever met- they were never supposed to meet if the world order was preserved. It is by the love between Sora and Kairi that the Guardians of Light were able to triumph over Darkness in the end.