Trans pan gasket..sealer or no sealer?? and other trans ???

That squealing you were hearing is called "Valve buzz"
It's the pressure regulator valve in the trans moving back and forth real fast.
The slipping AND the mud/flakes in the pan are from the direct input clutches being fried.
Gaskets that are folded up can be tacked to the pan with what you were told to hold it in place during installation.
Glad you have another trans you can put in.