I'd like to suggest an ergonomic mouse design where you have to grip it and close your fingers to get to the buttons, also if you use Teensy USB you can have more buttons and program their functions.
I can't really explain it in text but it will be very interesting to see it become reality and maybe the result will be better than the normal mouse design.
For now I'll be expecting the 2nd part of your current build to see your finger solder dispenser.
If you can't explain, I hope you can answer?
1)do you want something the size of a mouse or something to be the width of your palm?
2)how do you want it to move the pointer, laser or accelerometer?
3)wired or wireless?
Try to make a drawing of how do you want it to look. No one can make something by hearing since everyone's choices are different.
I'm not very good at drawing but I made a fast 3D model in Maya of how I think it would look like if it had a case but if it's a DIY I'd leave it bare circuit board.
So the fingers will bend to the under side of the top and reach the buttons and the thumb will have two more buttons where the green + is.
I took measurements of my hand to make this model.
About your second and third question... probably it will be wired and use the same method as the optical mouse because it seems simplest
