Andy, you're making my headache worse.

A ground would show a full circuit. The controller is looking for a ground through the sensor.
Think of the sensor as a variable ground. The hotter the sensor, the stronger the signal. Get it hot enough and it'll act like a wire going straight to ground.
A cold sensor has a weaker signal.

An unplugged sensor reads the same as a frozen sensor. There is no appreciable circuit. The fan will never come on because the controller is waiting for the engine to build heat.

It's either/or.
Run a switch and forget that a sensor ever existed.
Use a sensor and forget you ever heard of a switch.

Pick one.

No doubt you're going to ask for a sensor with a manual override for tinkering purposes.