JB I think you are overthinking and missing something. I recommend you start cold and think about this
Think about the sections of the problem in simpler terms and where the problem could be
1...The RED the ammeter and BLACK ammeter circuit along with bulkhead connector for those two wires, in which voltage drop can affect (2) below
2...The ignition "run" circuit which feeds the VR power, and the alternator field blue power wire. This also constitutes the sensing for the VR which is why ignition harness voltage drop (eliminating it) is so very important in Mopars. This of course also includes one more bulkhead connector terminal for the "run" ignition power
3...The charge path back to the battery from the alternator, however unless really bad, some drop from alternator stud (under charging load) from there to the BLACK bulkhead terminal may not be so bad
4...Something internal in the alternator wrong, not limited to, but including partially open or shorted stator and or diodes, partially shorted or intermitted field, and bad contact with the brushes and field.
5...Bad VR. These of course are solid state and can develop some interesting problems. Do not discount something simple like a bad connection in the VR connector, which can be difficult to "notice."
6....Defective battery. Batteries as well can develop weirdness, including sulfation If you have a known good battery out of another "rig" do not be afraid to drop it in, even temporarily in such cases