Ok. Thanks for the suggestions / ideas. After reading these, together with some pretty extensive online research, I decided that the MP6 really shouldn't be behaving as it did. So today I thought I'd do one last test cycle &, guess what?
It now works correctly! (?)
No crashes, power cycles or other tantrums. In short, exactly as it should be.
So, in each of the following instances, I get that little warning box pop up:
"USB-C Power is low. Battery power present, would you like to run on battery power?
If I choose
"YES" - batt gauge full-on green (presumably runs off 8AA sled)
"NO" - batt gauge orange with the USB symbol. (presumably runs off USB low power)
I tried each plug of the Y lead independently - fine
A standard USB A-C cable I bought in town - fine
2 different laptop pcs - fine
an Anker powercore 10000 - fine
I did give the MP6 a pretty good talking to the other night, showed it some scary looking test equipment which was gonna prod its insides, etc. Sometimes this does the trick. Machines can get above themselves...
So, now I just need to get out & hit record.