I had a long response but because the data base logged me out it wasn't posted, so I'll make this as quick as possible.
Good theory. The idea that people of Termina slandered the Goddesses rather than worship them like thier Hyrulian counterparts makes alot of sense considering they are parallels, but idea of Majora coming from hell sounds like fanfiction, it gave me some ideas though.
I think the
ST builders not only insulted the Goddesses but thier own guardians as well, due to your point about the Giants mask. Perhaps they were trying to show they could be just as powerful as them.
Perhaps they had another God, perhaps they were the Zelda form of Satanists who worshipped thier form of a Devil- Majora. the Goddesses should have an evil counterpart. Ganondorf's bad but he can hardly stand against a child of pure heart, let alone the creators of his world. Maybe another figure fits the description. Is Majora the Dark God?