Autophile and Qwerty, my driver removed the thermostat valve once but when we got work done, the workshop put a new one. Attached below is the list of stuff done back in 2010.
The radiator cleaning stuff you have mentioned has been repeated quite a few times indeed. The drivers can't figure out WHERE the water is disappearing even on the stationary car.
Your input on the head gasket particularly Honda cars facing the issue is of news to me. Question now is, WHAT should be done - everything all over again, head gaskit, thermo, what? Oh by the way, the heater stopped working too since these issues came up.