Initial diagnosis of both the throttle microswitch and the ECT is best done under the hood, first a visual inspection then with a meter. Later you can verify the signals at the ECU connector pins. (FYI. if and when you ever need to, be sure to probe into the side of the connector, not into the pins as the tiny fingers are easily bent and when you force the connector on may short and/or suffer permanent damage.)

You need to stay focused on the basics here. Begin by listening/feeling for the microswitch click as the throttle opens and closes. You can verify operation with a meter, but they rarely fail. If you've been playing with the throttle body stop screw (which you shouldn't have needed to) then the switch may well need readjustment. The wiring from there to the ECU is normally okay unless you've been playing with connectors at the firewall or ECU.

Following that, a bad ECT is well known for causing high idle issues. ECT's generally fail to open and you can end up with a wildly high idle of 1300 rpm or even higher. You can verify basic ECT functionality measuring resistance across the pins (using alligator clip test leads, be sure to keep the tips separated). For a basic functionality check, you don't want to see more than about 3K-7K Ohms with a cold engine. You can find the actual specs here in the 700/900 FAQ (read the whole ECT section for a full discussion).
https://www.brickboard.com/FAQ/700-900/EngineSensors.htm#DiagnosingECTFailures

When you start getting an idle much over 1500 rpm then I start getting concerned the ECU connctor wiring is no longer intact. Wiring under the intake manifold is especially sensitive to crumbling insulation in the mid-1980s 240 models. A short inside the harness is even possible. In that case things may get worse when you start handling the ECT cable. You will be able to confirm that when it comes time to check at the ECU connector. Also, if I recall correctly, with effort it's even possible to have the two temp sensor connectors switched. I have a funny feeling if you were to disconnect the ECT and try running it would be similar to what you have now.

Once you've got the ECT verified as okay then you go back to basic idle setup, starting with a good TB cleaning on the bench, checking the throttle plate stop screw at the same time, checking IAC functionality and using the thumbscrew to adjust base idle with the IAC test connector lead grounded to keep the IAC closed.

Interesting that you took the IAC apart. They're not considered serviceable and generally shouldn't be pried open, even for cleaning. Apart from grunge at the bushings affecting IAC operation, the armature wear grooves you found can be verified by tapping on the IAC during idle and noting any idle change. I once tried dressing out the ends of the grooves in the armature, but they're usually too deep to get any kind of lasting fix. You can try getting a bit more life out of them by opening the thumbscrew a bit to raise base idle into the 800-850 rpm range. The contacts then won't spend as much time at the step at the end of the grooves.

You can forget both the AMM and O2 sensor for now. You've got a basic idle control problem, not poor accelaration, engine rough running and fuel mileage. You will normally never need to touch the AMM adjustment -so don't even be tempted.
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Dave -still with 940's, prev 740/240/140/120 You'd think I'd have learned by now