Hi Tony:
I think you covered most of the bases. I am sure you made each of those changes one at a time to determine if any one of the was your solution. Apparently, so far none has solved the problem.
Since the voltage is not increasing as you depress the accel pedal it seem like the issue is in the throttle body positioning and/or TPS.
The steps for replacing the throttle body and associated parts are covered in this link and indicates what to do when the TPS voltage is not correct. You may want to switch out the throttle body spring internals, etc and/or use a new or different TPS from your other Mustangs. But even with a different throttle body you still need to set the correct voltage of the new/different throttle body.
I think a different or after market or stock throttle body you still need to check the voltage and then check the voltage changes with accel pedal.
john
link:
https://lmr.com/products/how-to-inst...-throttle-body