Suggestions for diagnostic elimination of possible causes?

I purchased used vehicle specifically to do some hauling, expected load approx. 3500 lbs. When vehicle was delivered, it lost oil pressure enroute home, down to 0, so it was left at my mechanic's shop. Sending unit(?) changed, he said it seemed fine.

Automatic transmission shift lever seemed sloppy (and could be "flopped around" by hand) and hard to move to me, and ignition switch sometimes "finicky."

Took off with trailer, within 4 miles, lost oil pressure again--gauge down to 0. No change in engine temperature, no noticeable engine noises like ticking or tapping or worse.

Ran several miles, stopped for two traffic signals. My sister heard high rpm from motor before transmission shifted at first stop. At second stop, I noticed the high rpms as I pulled out, and took my foot of the gas to encourage transmission to shift. Engine stalled, would not start up. My cousin noticed that when I tried to start, the hot battery cable (that runs to starter?) "jumped."

Truck taken back to shop on flatbed. Mechanic says he can't move drive shaft (?) and engine is seized. He says without dropping transmission (about 6 hours), he can't easily determine if problem is in engine or transmission.

Any suggestions? I took out a loan for this vehicle.