Some people are so quick to jump right into the conclusion that Iruma x Ameri is now somehow incest, even without any evidence. Guys, just let the chapters stack up and we will eventually find out the truth.
Iruma and Merize just happens to look like, with some slightly differences. Not every character with shorter, dark hair and big eyes are automatically an Iruma's relative.
To me, this arc is showing that both Ameri and Henri have similar tastes.
Now, about the theories, I do believe that this human girl is indeed Ameri's mother and Ameri is somehow half-human (Not sure how that would work, but if that's true, the author will reveal to us eventually). And I do believe that for Merize to stay in the Netherworld as long as giving birth to Ameri, I believe Henri used Transformation Loki on her to make her look like a demon, because It seems that no one in this story knows about Merize's true identity, as we can see in the flashbacks of Merize in the Garden, she looked different.
Or maybe in this timeline Delkira was still there and helped both of them to stay together. This wouldn't surprise me since the last time we saw Iruma using Transformation Loki, he looked exactly like a demon, for a short time, yes, but if Delkira was there when Henri fell for Merize, he could have helped her to look like a demon for way longer.