Overheating dilemma....need input.

I will chime in here with a thought. Just because the pump pulley spins freely does not mean that the impeller is still attached to it. The fact that the pump may have locked up and thrown the belt tells me the problem is more than likely the impeller as I have seen cases where the impeller had rusted completely off and others where the impeller shaft had broken inside the pump.
Hope this will help.

DVO