It seems as if the polarity has reversed thus the gauge reading backwards.

Re the starting if you have no fuel then the pump can't be working, to test this you should hear it prime when the ignition is switched on, I suspect yours isn't!

Probably not the pump but rather the relay but far more likely the wiring to it, thanks to your friendly furry intruder! You need to check you are getting power there (Can always test by putting power directly to pump and see if it runs) Yes the pump used is not very good much above standard power (Not sure the exact level) it's an Orion/Escort pump!

Martin