Hey friend! <3 your question is long, but my answer is simple.

It’s always okay to change the details of a character’s canon in fanfiction.

This doesn’t change, whether you’re making them a vampire, forcing them to work at a coffee shop, or changing their sexuality or gender.

There’s nothing sacred about a character’s sexuality that somehow makes it the only detail about a character you’re never allowed to change in fanfiction.

If a character is “canonically gay” someone writing fanfiction where they’re straight doesn’t change that, any more than someone writing fanfiction where they’re a vampire or a barista changes their canon.