There's a two wire connection atop the ac/heater housing (inside the cab) closest to the computer behind the glove box. The wire color escapes me but one has power. I also have a 91 gasser that this happened to. The whole HVAC was dead. No fan, no lights, etc.
The wire connector mentioned above...
Just FYI, I recently obtained my father's 1991 C3500 (7.4 TBI) and I plan on installing a Santa Cruz radio. Santa Cruz offers a direct replacement for the older OBS trucks that have Bluetooth and such. looks stealthy and is modern day. https://www.retromanufacturing.com/
You could disconnect the throttle cable at the pump to ensure that the cable moves freely through it's shielding. Other than that, I think you answered your own question with the shaft difference and pull angle.
Or, you could leave it and that would keep your son from straying too far from...
I used to have issues with my battery cables coming loose and when they did the ECM (computer) would not work and the engine would have a loud rattling sound, it was timing I suspect. Once I fixed the cables it was fine. So, what I'm saying is maybe it has something do with your ECM.
Best of luck