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... So, there are only a few options left -... the head gasket is blowing gas into the water channels,...

WHen this happens, you can usually see exhaust bubbles escaping the radiator(when filled to the rim) . and tHere are gas analysers that would indicate presence of combustion gasses...
Zero water from the exhaust is NOT a sign of anything....each gallon of burned fuel feeds the exhaust pipe with lots of water (MUCH more than a gasket leak) as a combustion byproduct...and you never see it in liquid form except when the exhaust pipe is cold and it condenses in the pipe....