This has been going on way to long. Fact#1, it had oil pressure when parked. Likely that bearings weren't damaged at that time. Fact#2 the oil pickup didn't fall off while parked. Fact#3 the engine does not need to be rotated to get oil to the gauge port. Fact #4 oil is picked up from the pick-up directly to the pump. Then goes in two directions. Through the filter or through the bypass in a loop. Remove the filter, spin the pump., No oil? If oil doesn't even reach the filter inlet my bet is the relief valve is stuck open. If it comes out there it should shoot out in a stream from the gauge port, like 6-8 feet if uncovered. Even worn bearing the oil will spray. There are other issues that could kill pressure, missing galley plugs etc. But you state it was fine when parked. The only possibility could be clogged filter, stuck relief valve, stripped drive. I would eliminate stripped drive as that's been verified you feel pump resistance. Why not just pull the pump, disassemble and inspect? As far as the squealing noise while running, bad news. What were formerly undamaged bearings may now be wounded.
