You are correct. The cruise control takes a vacuum from a reserve canister and two solenoids in the actuator and the potentiometer control the throttle position. It's described in the magazine link I posted above. It's extremely simple.
The diagnostic plug was used partially for check-out at the factory however regular shop tools including meters and lights can be connected to read back the status of various components and was encouraged for mechanics to use unless you had something like a Sun Modular Engine Analyzer. Those are pretty, but supposedly even the 80's era units still cost thousands.
The ECU is too early to have a proper computer interface. Like most cars until the 90's if you want to read out the stored fault codes you gotta use the trouble light. It's pretty bulletproof but I do enjoy how in my Tracker I can at least get realtime engine data without the need for an expensive external system*.
For AMC's,
this page tells you about how to DIY a diagnostic tool.
The stepper motor is that thing I found on the back of the carb that was unplugged. It's just a variable position electronic actuator that pushes or pulls two pins in the carb body that control how rich or lean the engine runs. The various sensors seen in the schematic above, including the O2 sensor which is playing a critical role here, are used for the ECU to determine how to operate the stepper motor. The transmission shop mentioned the car was running rich so I suspect that the pins are currently set to run the car excessively rich, then it was unplugged to hold the pins in place.....or the stepper itself is seized and needs to be serviced. The stepper Motor assembly is illustrated as component 11 in the below blown up view of the Carter BBD.
*Actually this used to be cheap but once Rhinoman over at Rhinopower Ltd. had the money get to his head he removed the free versions of his software, his DIY designs and locked down his $150 adapters so you were stuck with his garbage USB FTDI adapter. He keeps sending me DMCA notices for continuing to host the stuff he doesn't want you to see for free and telling him to jump in front of a train.