I did the jumper bit, but guessed at the wrong romex in the ceiling, which, inexplicably worked anyway. I've got two old wires coming in and two new romex. Have to make another stab at it in the light tomorrow.

It just looks as if it should be simple, but the house was originally lighted with gas, not electric, and things get kinda weird faster than you'd think. It all made sense when I wired it the last time, I just don't understand how it got so messed up.