You remove the diffuser to take out this little pin and spacer, and that opens up this path to the dif. I find me another beige gear and pin to seat on the other side of the diff.

After all that I can build a simple crank to go through, and connect to the gear, and now I can drive the whole powertrain smoothly and consistently. Bonus is that I can remove the crank, and replace the tile piece behind and preserve the looks!