[Richard]

One more thing to check, and if I am being too elementary, then forgive me. I would put my voltmeter on the pos and neg terminals of the alternator while the engine running and compare those voltages to voltages without the engine running. It is as close as you can get to isolating the alternator from the rest of the system. Perhaps you have done this already.

If you don’t want to reach in there with belts a whirring, then make two jumpers with alligator clips to attach with the engine quiet.