Sounds like you have done enough to confirm the fault is in the gauges.
100 ohms at the sender to gnd should cause the gauge to read about 1/2.
(E to F is approx 240 to 30 Ohms)
Powering the gauge with no sender will only cause it to read below empty. No damage.
I wouldn't go hitting the tank with you fist, but if you free the gauge from the panel, you could try giving it a bit of a tap, or rocking it side to side (with the power on) to see if there is any pointer movment. It may just be stuck at E.
It is sounding like you'll be needing new gauges though.
Paul.