Hi there,
I also think the mouse buttons might be an issue where you have them. One alternative would be to remove them, and place two small momentary buttons for the mouse above the trackball. I'm not talking about full size 1 1/8 inch arcade buttons, but much smaller momentary buttons. These would be just as functional, and would free up that space below your player 1 buttons and player 2 joystick. It would also allow you to move those buttons down as dogg0 recommended.
Using the player 1 and/or 2 buttons with the trackball will be fine (especially if they are moved down a bit). I don't think you would get much game use of those two mouse buttons as they are placed.. they're a bit too close to the ball I think.
One more thing.. you might want to consider a 4th button in your bottom row of buttons for both players, like in timoe's picture above. I was initially planning on 6 buttons per player, like you have. Then, I bought a partially completed MAME cab that already had the 7 button layout. I find that bottom row of 4 buttons is great for certain games, such as some of the NEO GEO games and the Dungeons and Dragons games. When playing fighter classics like SF2, I just use the 'regular' six buttons and never find myself straying to that extra button. That's totally a matter of personal preference though..
Looks good.. good luck!
-88mph