OK,
And the answer to this problem was---------
the motor box direction had gotten reversed in the settings.
How this happened is a great mystery as we hadn't even gotten into that area of the tablet/programming. Re-set to factory default cleared the problem (and so would have changing motor box direction in the settings).
My current working theory is that the tablet battery totally discharged and on re-powering did some funky stuff of it's own, likely in revenge for letting everything sit for a couple months!