Thanks, scottalexander. Drove around today, enjoying those "butta-smooth" shifts.
Plus1111, I'd relied on the suggestion that if the latch would seat, the sleeve was in the right position. Not so (obviously).
It's a bit surprising to me that just a few mm is enough to cause a non-connection. On mine, I'd guesstimate that it was at 6mm, and it worked fine - for 90% of the "training drive", then failed hard and long... never cut in and out. I can only guess that it wiggled another mm out of position during the drive.
Pressing it back in place with the plug in place was an utter joy, having taken at least an hour to wrestle that cursed plug into alignment with the tranny connector previously. ;-) It was a little frustrating to have to drain the tranny and remove the pan, but compared to messing with that plug... not too bad.