I take back what I said before. Iruma and Amelie aren’t gonna be cousins. They’re gonna be siblings.
That or, in a probably safer choice for Nishi, this humans gonna cark it, and Iruma’s gonna turn out to just be a reincarnation of Henri’s first love. Or it turns out she’s not even his first love at all, and he somehow meets his wife through taking care of this human.
We do know her name was Merize after all, and that’s not really a typical human name. Also human-chan here has some real noticeable eyebrows which don’t show up in the other flashback where we saw her, but that could also just be a later design choice, because in that flashback, we can see that Merize is wearing a hat that looks like it could either have horns poking up under the fabric, or could just be a horn-like design so Nishi could keep the fact that she was actually human secret a little while longer.
So yeah, this whole wall of theorising was actually pointless, because in the end it kinda still just leads to the conclusion of “Hey, could go either way, you know?”