Never seen a controller without a select button of some form so not sure what to say there other than I guess they exist then.
I'll be looking into putting that on another button then, and did you try using backspace with a controller plugged in?
Can't remember if it works or not right now but the game should at least switch to keyboard mode as soon as you press a keyboard button, it might not clear the controller button keybind though.
You mean overwrite an existing key as in if the same key is already bound somewhere else (not on the current action you're trying to bind) then just unbind that key for that action?
If so, that can probably be done and I'll put it on the list.
And finally, never actually seen someone use multiple controllers at the same time unless they're making games themselves and use them for testing so I didn't account for that yet.
I'll be putting that on the list as well and it will probably be added as a list you can scroll through and select the controller you want.