Cranks but won't start. Unplug ground at field spade on Alternator and it starts?

You stated this:

"This was the voltage regulator that had been on there for 20 years that died 3 weeks back and was replaced by the mechanical one from Napa:

https://shop.advanceautoparts.com/p...gulator-raa1052/12450617-P?selectedStore=2623

The local parts store says they can have one this afternoon."

As I'm reading through this long thread of troubleshooting, it appears to me that it all your problems started after putting in the new voltage regulator.
I would try putting in the NEW voltage regulator that matches the one that went Bad.