1. Will I be able to run the two wall shower heads off one 1/2" outlet?
One 1/2" copper line or one 3/4" pex line should work fine.
2. If so, will I need a pressure balancing loop to have equal water pressure to both wall shower heads or will just a T joint work?
So worth the extra piping but some of the plumbers here might not think it's worth the extra work. I would have it piped this way to be safe.
3. Will I be able to run all 3 showers heads at the same time without any noticable water pressure loss when in the 'shared' position?
If your restrictors are not removed you should not notice a drop in pressure.
4. Will this set-up even work, or should I just drop one wall shower head and be happy with two?
Best way is to try it first. Build the shower exactly like you want. Waterproof the floor and walls and then test the shower before tiling. If your home does not have enough water pressure or the effect is not what you want you can re-set and try another approach. I think you will be fine - if you leave those restrictors in place.
Once out the flow rates will change and your three fixtures might be demanding too much water.