[Sloan]

That was a safe assumption - I had checked the battery terminals and the cables, however this quick-disconnect had appeared clean-and-tight, and it was only on secondary evaluation that I discovered it was the problem.

As for the combination of symptoms being odd, even now that I know what is was I would not have thought a battery connection issue in the first instance. Strange shift function and Tiptronic fail warning while the car still ran did not immediately signal a connection problem to me ...