I reseated a cable and tested things again, being careful.
It turns out the multimeter is sometimes giving a bit of a delay before showing the 12v voltages, showing 0 while the wire touches 12v for half a second or so.
Anyway, the motor turns now!
I have not yet reversed it to unwind the cord, taped the relays back on, nor labeled the other board as broken.
I get the impression it might be nice to have test points of some sort to debug stuff. I have some wire-in voltmeters from another project. Not sure.