OP has ben chasing this leak for a couple of months...A UV dye kit has been recommended a few times. Has it been tried? If not I would...

I re-read your previous posts. You said you already swabbed your cylinders...But the excess pressure in your hoses tells me you probably have a head gasket problem. Have you tried a tester that tests for exhaust gasses in coolant? If not, please do so.

The fact that you also had a bad Aux fan (which you recently replaced) also leads me to this conclusion.

I suggest you find the leak, then worry about the thermostat..unless of course the car starts overheating.