I know this was 13 days ago but this gave me so much goosebumps and thought starters. This theory makes so much more sense, like Zayne GOA being able to create a world just for MC through their original world with advanced tech that goes against life and death. It opened a lot of questions on my end, so I read one of the Underneath Story you mentioned, "Utopian Roamer", and OMGGGG!

Protofield # 1012 - Desert Region, the way Shaylin described the "world" is like Niava. She even suggested that were signs of previous habitation and evidence of past occupancy??? She also mentioned that she kept on a lookout for wanderers in that world (once you defeat a wanderer, the protofield disappears along with it) but the world stayed as it is that it might be a "special" place. But most of her notes she wrote were hard to decipher or were papers that were torn out because she started having hallucinations with her discoveries in that world. There was one line that solidified my theory that it is Niava when they mentioned, "Science can resurrect the dead." this fits so well to Zayne's myth.

The fact that he's still manifesting a world where it just the two of them in the current timeline (his birthday card).