My car is back to normal after replacing the throttle actuator. The idle is now where it should be, the cruise control is working correctly and there are no codes.
So, to sum up what I did, I started with the cheap attempt and replaced the OVP. I then replaced the main wiring harness, which was in very poor shape. Then I replaced the throttle actuator, which was the pretty clear culprit from the beginning.
Ouch, that was expensive...
Experience is something you don't get until just after you need it.