Is using a remapping software such as AutoHotKey allowed, e.g changing the detonate button from 9 to F, or can it only be for changing keybinds to one key for a smaller keyboard, e.g having a 60% keyboard and changing F3 (not that it'll do anything) to something like 3?

Probably. Otherwise people who whole their mouse on the left side or people who use esdf to walk are doomed.

You can't do it via ingame, however, I do not have a problem with rebinding keys. Regardless, I would suggest using software that comes with your mouse or keyboard, if it has it. (For example, Razer software for their mice and keyboards)

Being able to rebind ingame is happening, we don't know when.

From Cish's Q&A