So in a way, is there a difference between headcanoning frisk/chara as girls or boys and headcanoning them as trans/nonbinary if it's all headcanon in the end anyway and everyone is equally wrong?

yes there's a difference because arbitrarily deciding that canon established neutrality is something that you get to override and is automatically more "debatable" than a character being established as male or female is a very obvious double standard.

interpreting them as nonbinary is only a way to translate their neutrality while assuming that the rest of their race thinks of gender as we do in real life. it doesn't change anything about how the character is referred to. rather, it preserves it. headcanoning them as a boy or a girl throws aside their neutrality entirely as irrelevant and "arbitrary". for what. YOUR discomfort?

while i might not think the characters in the text itself are trans, this attitude REALLY speaks to people's ignorance and weirdness around trans people in real life. because outside the game, real gender neutral people exist and they ARE, of course, transgender. someone who has gender neutral and nonbinary friends and is normal about it is not going to scramble to immediately fit any neutral characters back into the gender binary like a fucking 5 year old. because for someone who isn't transphobic, neutrality is just another way for human beings to exist.

anyway how many gendered characters are there out there? how many textually neutral ones, to compare? go play with another one of your billions of toys, if not retconning an entire character is sow hawd fow u

tldr: yes. kiss my ass