I'm right handed, so a trackball on the right side means use with my right thumb for better comfort and accuracy.

If you have to use the touchscreen, remove your hand from the trackball.
You won't find yourself needing to touch the screen and trackball together.

Putting the trackball on the back is a bad decision.
Anyone who knows a little bit about these knows how fragile they are.
After a few knocks, they can lose their sensitivity and become frustrating to use.

A clitmouse won't work, unless its on the rear and its very soft and very sensitive.
Again, its something that sticks out/exposed.

I guess a trackpad would work.
You'd need it on the back, since they require more size than a trackball for swiping gestures.
I just prefer trackball on the front over a trackpad on the rear.
There's no replacement for tactile feedback.

Hell, the keys could be mapped so that you can use the keyboard like a gamepad.
Use the trackball as the Dpad/nub,