Here is the "after" photo:
The mended bit is on the right, in shadow. As you can see, the original wall-builder subscribed to the "stacking" method of wall construction, whereas I am a "wedger." This was in part necessary because the "stacked" wall is three feet thick, and the two external stacked courses contain a heap of haphazardly piled rocks between them, so once the stacked course failed, the interior heap started to collapse as well. So I used big, flat rocks placed sideways for containment, and wedged the heck out of the bottom.
The wall on the other side of the yard needs mending too, but I think I might do that tomorrow.
At least I'm getting plenty of exercise.