Not a bad idea to undo the linkage, although if nobody has messed with them, it's hard to see how they could cause the problem. But maybe a drop of oil on each of the joints would help. Also a bit of penetrating oil on the accelerator cable, and check the slack: it shouldn't be tensioned at rest.

I've used brake parts cleaner to clean up the delivery valve area, but I avoid using it on the electrical connectors Carb cleaners are usually a bit more gentle. The thing you have to keep in mind is that the FSM says that to repair the rack position sensor, you have to replace the IP. So don't mess it up.

It's been many years since I worked on a 124, so I can't help with parts locations. But on the 190D, the computer, barometric sensor, resistor and OVP are located behind the battery (you have to remove the plastic cover under the cowl. The pin socket for the blink codes is next to the battery, it has a plastic cover.

Here's a video on the code reader:
https://www.youtube.com/watch?v=4vDwMf259Y8

And some info on manually setting idle speed when nothing else works:
https://www.youtube.com/watch?v=MgH0fIWNcSc