looks like you are off to a good start.
as for your control panel, you may want to rethink the trackball buttons. having them directly above the trackball is not really a comfortable place to use them. also, they are completely unnecessary. you can have any of your other buttons be mouse buttons. you can use your regular player 1 buttons for any trackball games.
for instance, my player 1 buttons 5 and 6 are wired up as left and right mouse buttons. they function just like regular buttons for any games that need that many buttons, and when playing a game in mame, they are mouse buttons. as for playing trackball games, i use the trackball with my right hand, and use my regular player 1 buttons with my left hand. this setup works great and doesnt require any extra buttons on the control panel
just a thought