Took her outside this morning, and the safety test without LiPo consistently threw I²C errors as yesterday. I brought her straight indoors, still powered up, both her and my piPad and ran the same test; she worked perfectly. Curiouser and curiouser.
P.S. Shortly after writing the above, I had a eureka moment in the shower: I remembered reading LiPos don’t work well in the cold, and even the Mavic instructions suggest letting it run for a while before take off to let the battery chemistry warm out. Next test then is to wrap both LiPos (batter bank and the main power) in bubble wrap, boot her up indoors, and then take her outside to fly. I’ll report back here anon.
P.P.S. It worked!!!!! I wrapped both LiPos in some neoprene foam (normally used for scuba suits), set everything up and running the code and therefore the GLL and PWM to keep the LiPos warm indoors. After a couple of minutes, I lugged everything outside, and she did two flights without a glitch. Roll on spring / summer!