I would be a bit nervous about hitting the valve with a heat gun. That cheap black plastic is pretty fragile. Is it possible to fill your fresh water, flame up your water heater and hit it with hot water from your external shower or use a kettle on your stove and use hot water? It generally does not take a lot of hot water to thaw something and less chance of a catastrophe. To thaw water lines I have ran a hoses up them, stuck a funnel in the end and poured hot water in. Goes real fast.
I dont really think it will hurt anything if you leave it frozen and if you break the valve you really will have a poopy situation. Pipes and valve are already froze so if they are not cracked now they should be ok. My only concern would be the tank expanding esp if vent stack is froze. Might wanna throw another jug of antifreeze in and stir it (yuck) with a stick if you can.
Best of luck. Maybe put a heat trace on the pipe and heat pads on the tanks for next year.
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