Awesome!
It's good in so many ways that I can't tell. It really feels great.
The only drawback I see:
Having to use the mouse in the menu is okay and you take advantage of that to allow for custom keys, which is great. But from that point, when you finish a game you have to use the mouse again and it's not optimal :D
You could maybe make it so that once you finish a game, you can easily start a new one with the same options (solo/coop + chosen button). But to get back to the menu you press the button differently.
I have to play it again, bye.