Gauge or sender

Well, you are assuming that when the tank is actually full, the sender is actually positioned at the 9.5 ohm full position. Without filling the tank, you'll not know. But I understand why you don't want to.

One of my next suggestions was to bend the float arm so that the resistance is lower lower for a given fuel setting. Bend the arm downward to do this.

But you are still shooting in the dark. It's been suggested twice that you get some resistors to check your gauge. If your sender out of the tank can be set to read 9.5 ohms on one end and 73 ohms on the other, now would be a good time to use the sender as a variable test resistor. Take out the sender, set the sender resistance to each of the 3 specified values in turn, and plug it into the gauge circuit and see where the gauge goes. At this point, it is a pure guess if the gauge is good or not without testing. You obviously have the right skills to do this.

And check that IVR output voltage as suggested so that can be eliminated. The substitution test is good, but not proof.