It honestly at this point looks as if it is a grounding issue.
You have almost gotten all the parts out and on the ground at this point, the only thing you have left to try is hooking a wire up and grounding the gauge directly to the sending unit (bypassing the frame connections) and running this connection to the battery ground.
At this point, you have already dedicated wiring from the gauge to the sending unit, no?
Grounding directly is the only part left to try.
note: good form in grabbing the gauge that matches the sending unit.
See pix for wiring.