Final update, because I'm just absolutely stumped by whatever this issue is/was and give up...
Tried 2 different multimeters testing current today, Mac's proper Fluke clamp version, and my own cheap version - both reading 0.0 or 0.01 respectively, regardless of which AC unit was connected.
View attachment 106682
View attachment 106683
So, apparently there's no more drain, even though nothing has actually changed and the battery is losing on average a steady 0.07v a day.
Absolutely no idea why the car has failed to start multiple times over the last few months, as the battery itself holds charge perfectly, and the alternator appears to be charging it...
Good luck to anyone that ends up here with a similar issue!