Hello and welcome.
The coolant light will come on if the coolant is low, or if the level sensor is faulty. If you are positive the coolant level is fine, then the sensor will need to be replaced.
For the wet rear seat, this is most likely due to a bad door seal. This can be verified by power washing the car with an assistant inside who can watch for water ingress.
If the car is not producing sufficient heat to defrost, then you may have a bad heater valve.
It does not sound like you have a head gasket problem at all.
I hope that is helpful, please let me know if you have further questions, or if I can clarify anything for you.
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Many thanks and happy holidays.
It should be easy enough to check for coolant leaks with a pressure test. And, you can buy an inexpensive kit at autozone that will test for gases in the cooling system, which will confirm a headgasket failure. This should resolve any uncertainty.
In the back seat, this is most likely water leaking in somehow, not coolant.
My diagnosis on the rear flood remains as water leakage - door, window or sunroof leak.
Overheating and misfire codes can indicate a bad headgasket. Sometimes no codes are set, depending on the nature of the failure. The autozone test kit is a cheap and effective way to verify.