Here are my thoughts:
First of all, regarding the angled/nonangled debate on player's 3 & 4:
I used a 38" panel with players 3 & 4 on the extreme upper corners. In that position, I tried both regular and angled controls. P3 and P4 were UNPLAYABLE with the regular controls. I had to angle them. Since then, I have come to the conclusion that if these two positions are further from the monitor (closer to the P1 and P2 positions), then angling them probably wouldn't work real well, and you'd want to go regular. So I ernestly believe that the answer to this debate is that "it depends." Your best bet is to design the position of the joysticks and their buttons so that those two sticks can be mounted either way, then try them both. Once your machine is up and running, you will discover that one definitely works better than the other -- but it varies based on too many factors, so until you try it, you won't know.
Secondly, do you really need seven buttons in the P3 and P4 positions? Look at the games you are going to be running. I don't know of a single 4 player game that uses more than 4 buttons. If you reduce the number of buttons in those positions, you'll have more flexibility in how you place those players.
And finally, I would get rid of the three buttons above the trackball, but that's just me. They're useless for gameplay, and they'll just confuse people who are using your machine. If you really want to map a couple of buttons to mouse functions, then you could use just about anything (like three of the P1 buttons -- or just 2, do you need 3?). Of course this means that you'll have to enable the mouse in any game that might use those buttons. One solution that I used was to build two buttons onto the underside of the control panel, and assign the mouse buttons there. They were there for me to use, but the average person who used the machine had no idea they were even there (nor did they need to).
-Jeff "etumor"