It seems like they've fixed the issue with not being able to rebind certain keys although I'm not sure when. I was able to rebind T and Y as well as move the change weapon button to another key and rebind that key. For T and Y the menu doesn't unbind the keys automatically so I had to go to the bottom and assign them new binds and C wouldn't let me bind another key to it until I moved the weapon function key to another. It's weird because every other key will just overwrite if there are conflicts but these don't. I only tested on the steam version but I'm assuming the epic version would be the same. I would edit the wiki myself but I'm not entirely sure how.
Topic on Talk:Borderlands 3
I would have to look, but last time I checked, remapping the menu navigation keys such as Z, C or Tab was not possible.
You're right menu keys can't be changed but at the very least they fixed the issues with keys used during gameplay. I'm so used to games locking in menu keys that I didn't even think about it.