Holy cow, I wrote a book!
Since my original article explaining
how to get Windows Media Player to single-step a video,
I've learned that there's an even easier way.
Backward-stepping is dependent upon the codec;
some of them will go backward to the previous keyframe.
The person who tipped me off to this feature:
The developer who implemented it.
Sharing a tip does not imply that I approve of the situation
that led to the need for the tip in the first place.