I don't have a manual handy, but where does the pump take suction from the sump? Is there an internal dipper tube with a screen on the suction line? With all of the troubleshooting you have done (which seems well done) it seems there is a considerable restriction in the suction, causing a much lower than usual pressure in that suction line, effectively boiling the oil due to the lower pressure (cavitation). Is there a way you can check the suction pressure, and compare that to typical suction pressure of the same type unit? This would let you know if there is a crushed dipper tube, or clogged suction screen in the sump, if there is one, or any type of restriction.
Also, I know that a number of these tractors hydraulic pumps take suction from the sump AND the power steering pump. Maybe there is something happening there?
This may have already been done, but I wanted to mention.