At this point, I would pull all my spark plugs out and compare them. If one of them is steam cleaned and looks like a new one, you are getting water into that cylinder and you have big problems.
When a head gasket leaks or a head cracks, the leak goes both ways. Water gets into the cylinder and steam cleans it, and exhaust forces its way into the cooling system and pressurizes it such that it pushes water out through the cap. The way that a mechanic in a well-equipped shop checks for leaks is by taking his exhaust analyzer and checking for carbon monoxide at the radiator cap when the car is running. If he detects any, there is exhaust forcing its way into your cooling system and the head has to come off.
If you pull your spark plugs, be sure to take the tray that holds your cabin air filter out of the car before trying to access the rear spark plug. you can get the plug in and out with the tray in place if you happen to have the exact length extension to fit, but it is easy to cross-thread the plug working under the tray.