So ok, I think I have found a culprit...
First, I tested for phantom current draw. There is minimal, .013A.
Next I tested this car, with my TTG charging system. Even loaded up, Highbeams and fan running, my TTG at idle supplied 13.5V (via jumper cables).
THEN, I tested this car with the good battery, and using the hand-throttle to keep the revs up. Loaded up, with battery connected, registering about 10.5V.
Disconnect the battery and the output drops to 9.8V
I don't know precisely how it works, to explain why it appeared better with a crap battery.
But this would seem to support the theory that the Alternator (despite being rated at 120A) simply cannot support any kind of load, and hence running lights and fans, even whilst driving is draining the battery.
And yes, to my shame, this is a POS ebay Alternator. Lesson Learnt.
Can anyone recommend a good brand and supplier of an Alternator?