Cooling System Woes...Need Help

Depending on the brand the stops leak type products tend to become
liquid at high temperatures and harden when exposed to air.

You could try boiling the thermostat housing in a pan of water and see
if it softens.

Some of the housing I have seen over the years are really rough on the
inside. If it does not bind the thermostat just put it back together.

Flush the system completely before replacing the thermostat and refill
with the correct coolant.