I don't understand the thread title. Why is it possibly a seized engine?

Skip the sealant. You can't call someone a shade tree mechanic and then propose using sealant

If the rad cap is loose there is no pressure. Tighten it up and see if you have a coolant leak. You can definitely overheat with the cap loose since the boiling point is lower with no pressure

The spiky thing in the footwell is the final stage resistor. It controls hvac fan speed, not cooling. Your ac isn't connected to your cooling system except that it can absorb heat

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