I haven't received any responses.
If I finally go with the third control panel I'm going to wire it as 3 and 4 joysticks. Unless someone here gives me better advice from experience. Because so much has to be remapped anyway, and a four player cab might work for Gauntlet, then map the 1 and 2 joys at a 90 degree angle (Up = Right etc).
The reason I say If is because if I can get a small trackball on one vertical end I might not add the third control panel since Mame can split-screen co-op games in vertical mode. I would be giving up Robotron, Battlezone and a wider screen for horizontals but gaining less work, money and cabinet footprint.
As for the RGB buttons; I'm assuming all four sets of player control buttons are the same when it comes to fire, thrust etc. (the main reason I want them lit up, so new players know what button does what) so all those buttons should be the same, unless you want a spectacular unique attract mode. But I'm also assuming that if I run one lead from the LED controller to four #1 fire buttons for example the power would be 25% for each.