Wait what? Does Ameri know Iruma's human????? I don't recall

Yes she does

Has known ever since she laid eyes on him, and the fact he could read the manga just confirmed it. (It's in a language no demons would know, since it's from Japan)

She's just tried to not think about it to maintain plausible deniability.

Yes, Ameri is aware, but Iruma does not clearly know that Ameri knows he’s human. Honestly, the moment he read that book, it was already pretty obvious that he’s human, so it feels like a open secret between them.