Just as a little more diagnostic evidence: So far there has been no other issues with this mess. Still got the accessory-load-with-engine-off bit at every time it is triggered. It is my daily commuter though going to and from work every weekday about 5-10 minutes each way. Starts right up, battery has no issues holding a charge. Just that single VERY weird issue with the battery voltage (according to the PCM/voltage gauge on dash) rapidly draining with the engine off and accessories full blast but no other indications that there is any change in charge (HVAC blower has no audible change in speed, headlights at the same brightness, engine kicks over with no hesitation even with the headlights left on).

 

Reset the instrument panel.

Hold down the reset on the mileage and turn the key on till it says reset and let go (about 30 sec). It will go through a series.