Code P0122

Well, this is getting kind of crazy. Did you disconnect the cable from the throttle body yet?
Please do that. 1.63 volts at closed throttle is way too much. It should be below 1 volt at closed throttle. Your throttle plates not closing all the way is causing that 1.63 volts and that is incorrect. Disconnect the cable and see if the plates close all the way.
You may have a faulty cruise control module or something holding those throttle plates open.
Disconnect the cable before you do anything else. Then take another reading on the dark blue wire at the TPS with the cable disconnected, it only takes 5 minutes to unhook the cable, especially on this car.
We're just going around in circles here. You have to figure out why its not closing all the way.
0.03 ohms is barely anything, disregard that for now.
Concentrate on the throttle body.
The gas pedal could be binding up the cable as well keeping the plates open, so you have to take it out of the equation.

Okay, so the gas pedal is binding up or the cable is. 0.63v is perfect for closed throttle. Did you start the car with the cable disconnected? That's the big test factor here. You have to try that and see if the idle comes down. If it does then you've found then issue. It seems your gas pedal or cable is holding the throttle plates open too much at idle.

Try running the car with the cable disconnected before you go messing with the IAC. The IAC is not setting any codes. So stick with the TPS for right now. Don't do any unnecessary replacing of parts or testing right now.
Start the car with the cable off. That 0.63volts is an acceptable reading.

Please don't go get another IAC yet, just start the car!

Don't be throwing parts at the car. We don't do that here. We diagnose, then replace.

If the idle is down with the cable off, then we will find out what's up with the pedal. Remember the car is 20 years old, The IAC would set a code if it was having voltage problems.

Yes, unfortunately you ruined the IAC by pulling the pintle out, that's something you should never do. Do you have the old one you can put on there? or get a new one.

So we're at, the throttle plates are fully closed, the TPS reading is 0.63v with them closed.

So lets leave the throttle cable off for now so those plates stay closed.

If this is where we are at, and you're still reading 0volts on the Signal wire at the ECM (Pin 66) dark blue wire? The 5 volt ref is good,? And the ground is reading 0 volts going to battery negative?

Note: If that's our case here then its obvious there's an issue with the Signal wire somewhere between the TPS and ECM.

That's what you're chasing if all the above is correct..
There is a connection (p100) which is the Harness plug going through the firewall.
If you have followed that wire all the way to the firewall and found nothing wrong, then you need to start checking inside the car for a broken wire somewhere.

I'm really hoping you had a good ground connection with your meter when you were checking the ECM pins.

Another method is to use the ECM ground while checking Pin 66.
So put your black meter lead on Pin 16 Connector 1 (ECM ground black/white wire)
Red meter lead to Pin 66 Connector 2 (TPS Signal dark blue wire)
Since you didn't get any good readings with just the key on, do this engine running.
Just be extra careful with the engine running doing this. Have everything ready before you start the car so you don't accidently short anything out.

If you get the same TPS reading of 0.63v at the ECM now. Then you have a vacuum leak somewhere.

I'm doing my best to keep track of everything you've done so far,
If I have any of this incorrect let me know.

If you suspect a vacuum leak, take a spray bottle full of water and spray around the intake manifold and vacuum hoses with the engine running, you will hear the water being sucked into the engine. Some guys use brake cleaner or carburetor cleaner but it's very flammable and I don't recommend you doing that unless you really know what you're doing. It can easily start a fire. I'm leaving a guide below to vacuum leaks.
Just don't spray the spark plug wires, you can get a shock very easy.

But we first need to know for sure about the TPS signal getting all the way to the ECM.

https://www.2carpros.com/articles/how-to-use-an-engine-vacuum-gauge

https://www.2carpros.com/articles/engine-idles-too-high

I would really like to know if you are getting no reading on Pin 66 of the ECM, while you are getting a reading at the TPS dark blue wire. If the car needs to be running to get a reading, then run it. But the code p0122 is there for a reason, There are other TPS codes it could have set, but its a low input code and you're reading a low input. I would retest again.
This would explain the code and the symptoms. I know you have done a lot of testing and are probably exhausted because of this. But if these 2 readings differ. Then we can try somethings to verify a broken wire.
We can even run a new wire to see if the idle comes down.
And as for grounding your meter correctly, you can always use a long piece of wire and run it to the battery negative while checking circuits inside the car.
I do this a lot. Then I know I have a good ground on my meter.
You have done really well so far, and I think you should keep going and figure this thing out. It will help you in the future to solve other issues with your vehicles and save you a lot of money.

I think I know what happened, so before I tell you my thoughts on this.
When you reconnect the throttle cable, if it is still keeping the throttle plates open a little, then you may be a able to adjust it at the throttle body so the plates are closed when the gas pedal is fully up. You want to make sure that its all good in that area. Also I want you to make sure that the connector for the TPS doesn't have loose pins, just make sure inside the connector that the pins are not spread apart, you can check them with one of the T pins if its the same thickness as the pins on the TPS.

Now on to what I think happened, With you messing with the ECM connector as i'm assuming you did yesterday and today, I think that Pin 66 on connector 1 has a pin fitment issue. And with you moving those connectors around and kind of fighting with them, the Pins made contact with the ECM pins or you moved a the Signal wire away from a part of the frame where it was touching.
The only problem here is that you may run into other issues if other pins are loose. After years of those pins heating up and cooling down, and vibrations, either they loosen up or the ECM has a bad solder joint inside on the circuit board and again you're moving it around it made contact.
I had some suspicions this might be an ECM problem. You may want to move the ECM connector around again and see if you can duplicate the problem again and inspect the wiring in that area,
I know you want it fixed and it might be for the time being, but if its a bad solder joint inside, this will happen again.
What did you do exactly in the time between when it was having the high idle problem and now that its idling normally? Was it only messing with the ECM connector? Or was there anything under the hood you did as well?
If this problem doesn't come back soon, you're going to want to have the codes erased from the ECM, because there's probably other codes from us unplugging and testing things and it's good to clear the memory after a repair so the ECM can relearn its fuel strategy.

The throttle cable should have an adjustment on it either at the pedal or the throttle body if it's too tight holding the plates open.

There should be a nut you can loosen up to give the cable some extra play. Can you send me a picture of your throttle body close up so I can which kind you have. The picture i'm leaving is a basic setup. I wondering if you need to replace the throttle cable. It may be binding up somewhere. Or there is a problem with your gas pedal.
Amazing huh, the problem may have been the throttle cable this whole time. But I would give the gas pedal a real good inspection too, see if you can pull it up or if is staying up by itself.
If there is no adjustment at the throttle body or on the pedal, then the cable needs to be replaced. If it is binding up that bad, its going to be a safety concern driving. You don't want your cable to get stuck and lose control of the car.
The 2nd picture is what I found for GM throttle cable. Is this what your setup looks like?