Of course, you cannot be certain, but I don’t see why you are trying to criticize people for only discussing future possibilities that have been brought up in text instead of future possibilities that have no textual evidence. There is nothing in the manga to imply genderfluidity, while there are multiple scenes that hint towards being a transgender woman (“Maybe I want meant to be a girl all along?” + “A real girl… I’m trying to be one”). Perhaps it is reflective of your own biases that, despite textual evidence pointing only in one direction, you seem to consider genderfluidity to be an equally valid reading?
I think this is a really good paragraph that cuts into many of the core issues I take with some comments, both here and on other sites. What really confuses me though, is why you then start using they/them for Momiji/Kouyou in a later comment? They/them aren't a truly neutral pronoun, and I would argue that in this case, that's degendering.
You say that "if you ignore the parts where the character states that they prefer being a girl, you could formulate a much stronger case for the character being agender" but I don't think there's reason to ignore those.
it's not often one can do stuff to only pass some of the time.
Once you learn how to pass it's really not hard to go back and present how you used to. Especially as transfem, since the gender markers for women are far, far stricter than those for men.