Heres an example. My oliver 90 was delivered without provision for a thermostat. It had a radiator blind to regulate the water temperature manually. It was later changed to thermostat (the top water manifold was changed, and I still have it).
Heres another. I wondered why may car heater was cold one morning and the engine was running better with the choke in operation. Thermostat was stuck wide open.
Heres another. Most early iron only used thermosyphon as a means of cooling, no pump and no thermostat.
All those examples did not reach proper operating temperature without the thermostat (bar the ollie 90 with the blind well adjusted).
As to the thermistor problem, you likely have a short to earth in the wiring from the guage to the thermistor.