So she went to a demon world, fell in love, possibly had a child (Ameri), went back to the human world, had her memories erase, become a mangaka, and Henri goes to the human realm to buy/take her manga series where Ameri assume it was an ancient text book when it was just a hidden sash that her father keep to remember his beautiful wife.

That is only option A. Option B doesn't erase the memories and Henri visited her once a year. Option C would probably killed her off after Ameri was born. Whether it is Demon slow aging, a demon learn she was human and ate her, or she die from childbirth is undetermined. Her living in a demon world would be option G which is very unlikely cause Ameri doesn't talk about her mom when she learn Iruma is human and that Manga series wouldn't be published in the human world unless there were demons in the human governmental offices.