I still think that Amelie is half human, rather than half deity as some have suggested. Narnia's threat of "human deportation" at the end of 369 while seething at Henri implies its a threat towards him specifically, through Amelie.
My theory is that Henri hid his human wife in the Deity Realm to keep her safe, possibly because she wasn't safe in the Demon Realm, and this trip here will finally reunite mother and daughter, at least temporarily. And also to explain that there could be a difference in flow of time, given that the manga that Iruma was an assistant on had dozens of volumes yet were things Amelie considered old and forbidden at the start.