This is a lot of great information, Dave.
UPDATE ~~ It now appears that the batteries had been discharged too much, to permit adequate current delivery to the 12V regulator. I directly measured the battery voltage off the terminals, and saw 21V !!! I’ve never seen it that low before. For the entire previous night, the battery charger was left “ON”. I can only guess that somehow, its connector was not properly seated in place. I am trying a second charge-cycle now.
QUESTION ~~ When external battery charger power supply is connected (through the 3-pin DIN), is the power regulator onboard the Magni flexible enough to simultaneously charge the battery, and supply the Raspberry-Pi / Motors / etc ??