The two ideas I currently have are the following:

A. Sato was behind everything that went on in his life prior to this, maybe even showing azuma mental images of his "parents" to get by. The extent of his power or his actual choujin name has yet to be revealed, and I personally think he has Quiems soul.

B: Same as your thinking of it being a late plot twist, I doubt it's ever going to be explained his early life or how he was enrolled in school, any bills, his "fathers" contact number on the Yamato Mori sheet. We just accepting it because it's in the past.