you can set 1 - 9 keys to your favorited items by opening the favorites menu (default Q) and then hitting the number. I'm guessing same can be done over the numpad.
as far as I can tell there's no way to change that functionality in-game.
default enter key is E, you can remap it.
you can remap the default WASD "arrow keys"
if you meant something else, please elaborate on your question.