Minimalism is nice, but within reason. Personally, I believe that the most optimal Number of buttons is 8. (Just like in the NES.)
You have D-pad ("4 buttons"), 2 main buttons (A,B) and 2 additional buttons (select, start).
Games are mainly used only 6 buttons (D-pad + A,B). But if you want, you have two additional buttons. You do not have need to use them, but they are.
At the moment, we have only 6 buttons on one player (Total 12). You are talking about 16 buttons ... But [id: 6,7,14,15]doesn't work at the moment (I checked). And all of my ideas about the use of the 12 keys is just a suggestion how to get around a limit of 6 buttons for a single player game for the current momnet (Of course there are problems with the gamepad. ).
P.S. What if the first player wants to play with the keyboard and the second player with the gamepad? (At the moment this is impossible)
P.P.S. Сustom Key mapping is a great thing. It doesn't matter what it is (Keyboard or a gamepad or the onscreen buttons in the smartphone). I hate when I am forced to use the "Motion Control" with my right hand, and "main buttons" left hand.