I am a card holding redneck, but after all you have done.
I would install a mechnical oil pressure gauge if you do not have one, accurate reading.
Then change oil, install a wix or napa gold oil filter, shell rotella 15w40 and 1 qt of lucas oil stablizer, that should increase the oil pressure some.
this fix will only cost you a oil change and you need the mechnical gauge anyway.
If the oil change does not fix your problem then you need to have another mech look at the problem, before you lose the motor.
Lack of oil pressure is the fastest way to ruin a good motor.
Good luck and let us know what you find.