Do the voids and valleys on the back of the face need to be there for clearance reasons or for any reason ?

Only enough to make room for the translucent lens to fit into it. The rest was just for making the plastic mold easier to work with, standard plastic-casting stuff.

Does the slight raised lip on the front of the face have to be there ?

I don't know what you mean. What lip on the front?

What about the slight raise lip around the 4 buttons ?

The openings for the buttons have to be carefully shaped to fit the buttons, and the face is responsible for holding the buttons in place. So you would have to do some experimenting to get the right shape around the buttons.