I think you guys need to give the Japanese some credits. Yes, some of them put questionable incest into their stories, but even their readers over there didn't accept such nonsense sometimes. Some of you also jokingly (hiding behind it mind you) protest if there are no incest. There are discords about it in the Oshi no Ko's fandom
On that note though, Merize to me is definitely a human. In chapter 326, where Henri reminisces about his wife more up close. You can see she wore a hat shaping a pair of horns up top, we can't see the shape of her ears, and when she smiles, there are no fangs. It's as if she deliberately dressing up to cover that features. She also doesn't have a tail! Granted, not every demons have one. But the horns, ears, and fangs, are the most common features of demons, yet hers are hidden away.
She's also definitely the author of Amelie's favorite manga. Every time we are shown Iruma's life back on earth, especially when he needs Amelie's mangas, we were never shown the mangaka, only their assistants. She's always behind the door, where she couldn't be disturbs. The human in this arc also is good at drawing, her drawing is one of the factor that makes Henri falls in love with her.
We needs to gives Nishi-sensei her due respect. She's a great story-teller, the plot twist she makes actually make sense. And we can conclude to that assumption based on the crumbs she gives throughout the story. Not every plot twist need to be something out of left field, something they hide deliberately to pull the rug under your feet, that's not a good plot twist to me, this is not Disney.