Thanks for the suggestion.
In the meanwhile, I believe have good (good/bad) news:
Trying some (small or freebie) movies for DL, I have not have the problem with others.
But I hesitated to throw away $4 on "real" iTunes movie.
However, a round trip to Apple brick-mortar store would have cost 4x that in gas, so...
I re-rented the troublesome iTunes movie. Now *both* appear in my "purchases."
But the *second* version played through just fine.
• Some videos (or some DLs) have data integrity issues.
• The "Videos" app does not (always) recover properly from such bad data.