Have you tried to just test the system by replacing the suspect parts with say a 100 ohm potentiometer, some would call it a reostat, and an ohmeter. You could hook the pot directly to your gauge and see what what ohm value drives it to full and empty. Then hook the pot in at the gas tank and repeat the test. Then measure the resistance of your sender with the ohmeter at the tank. This should answer some of the questions you have. Good luck.
