These are all valid thoughts. My only concern would be the fact that the engine doesn't respond to the rating of the installed thermostat. If the cooling system is functioning as it should, and you install a 180&deg thermostat, that is the approximate temperature the engine should operate at under normal running conditions

If you try several thermostats of differing temps, and the systems doesn't respond, my first thought would be that the prime cooler...the radiator, doesn't have the ability to cool properly, and at some point, you are going to find this out the hard way. If the rating of the installed thermostat doesn't regulate the operating temp of the engine, what does?

Stuart