Code P0122

Yes, I understand it's a lot to take in, it's just as frustrating being on this end not seeing the car for myself. There could definitely be more than one problem here. But I think if you try another TPS, at least we can completely rule that out. They aren't making decent electronic parts these days. The car is 20 years old, so almost anything is possible by this point.
I would try another TPS, give the throttle plates a real good cleaning. And i'm not sure if you tried putting your finger over that IAC hole in front of the throttle plates and if that brings the idle down.

You can back probe the dark blue wire with it running and see what the voltage is doing or if it does that lose of voltage like you were describing. If that's really happening, it can be a bad sensor, a lose of that 5 volt reference, a bad wire somewhere, or another sensor is shorting out bringing the 5 volt ref down. You can see why its tough to figure this out from here.
If you're back probed at the ECM on that Signal wire and there's no Voltage increase with the pedal. That's our issue.

For the code p0122 to set, the ECM has to see the Signal from the TPS drop below 0.10-0.17volts. For more than 1-6 seconds.
As for your grounding question, you can try the door for a ground, but you have to make sure its really a good ground.
So if you're at the ECM, try the door as a ground and check the ECM (orange wire) power feed. If you have full battery voltage there, you're good.
Sorry this is so frustrating, if you lived down the road I would have driven over to your house by now. But diagnostics is tough work if you haven't been doing this for 20+ years.
I figured trying a new sensor would lessen some of the headache.

I just know this code sets because of an off voltage reading. When I look up the criteria for the code there's a list of things it can be, plus a voltage variation for the TPS. Ill show you the list so you get the idea.
Do you hear anything that sounds like a vacuum leak, it would be a loud air rushing in type sound, but it may be difficult with such high rpms.
I'm putting up the Diagnostic Procedure from our information source, but it calls for a having a scan tool to watch the data and check the freeze frame info as well,
You can try this method, i'm just not sure how far you will get with it, it talks about jumping the 5 volts to the signal wire, and if you do that wrong, you can fry ECM parts. So I don't recommend it.

I figured it would be hard to notice any changes with an engine revving up 4,000 RPMs, it would be hard to hear anything. If you did feel some vacuum, that means the IAC valve is open somewhat, if it was stuck open the idle would come right down and probably stall the engine out.

What you're describing with the idle going up and down, is the ECM hunting for the correct idle but not being able to achieve it.
If there is some vacuum at the IAC that's going to be expected, since the ECM is using it to try to correct the idle condition. But there shouldn't be a real strong vacuum.
Ill bet that IAC is opening and closing back and forth trying to correct for something.

Are the throttle plates completely closed? They really should be, if they're not, there would be some slack in the throttle cable. This would indicate the plates are staying open due to carbon build up. But I remember you saying the cable was tight at one point.

This just keeps bringing me back to that voltage drop off on the TPS signal wire.
The ground reading was ok and you had the 5 volts it needed.
Unless that Signal wire is rubbing on something metal and is shorting out to the frame or engine block or is corroded in a connector. That would pull the voltage low for sure.
I would check every inch of it if possible, all the way back to the ECM.
Sometimes you were getting a reading with pedal and sometimes not it seems.
Try a sensor so I can get that part out of head.

It cant hurt, we've been at this all week. Something has to show here eventually.

Okay. Well, I'm glad you got all that done though, that helps rule out a lot here. How much is the throttle plate open? And if you were to loosen up the cable do you think you can get it closed all the way?
As for the EGR, its probably got carbon build up, taking the valve off and cleaning out the ports is always a good practice in general. The ports that go inside the intake manifold can be very hard to clean out sometimes. Depending on the design of it. Thank you for trying another TPS.

Checking that dark blue wire at the sensor is a good idea, either that voltage signal is incorrect, or its those plates staying open when the ECM thinks they're closed. I'm wondering if the bearing that holds the plates is binding up or something along those lines,

If you loosen up the cable and it doesn't close and you cant clean it anymore, Id look into a new throttle body or at least at the auto parts store have them show you a new one and check the difference.
But we do need to verify that voltage signal.
The throttle plates (butterfly valve) aren't controlled by anything except the cable on your car. Looking at the wiring diagrams here, its just the cable controlling that. Possible the cable is binding up,
You can pull the throttle open with your fingers and unhook the cable without having to do much else. See if that closes those plates. Don't be afraid to take the throttle body right off the car and investigate.
You're right there man, I know this has been a huge pain all week, but you've done good work. Doing the TPS and IAC at the same time was a good idea, id leave the new ones on there. Then you wont have to worry about those in the future.

Okay, from here, disconnect that throttle cable first. See if the plates close.
If the cable is binding up or for some reason the cruise control is holding the cable keeping the plates open, we have to know the plates will close on their own.
Then we can determine if its the cable or something else.
Do that first tomorrow or whenever you have a chance, Just pull the throttle forward with your hand and you'll see you can slip the cable off the throttle body.
If the cable doesn't do it, check the stop screw on the plates, if someone has messed with it.

So pushing on the pedal, there's no response at all? Has it always been that way?
And 1.6v at idle is too high for the TPS. But if the throttle isn't responding at all. That's a whole different story. The smell might be the engine running too rich if you don't see and oil leaks.
The throttle response is your issue, if your pushing on the gas and nothing is happening with it running, its either gone into Limp Mode, or you may have a bad ECM.
Under the dash on the driver side, look for the ECM. Its going to be a big flat silver box.
Use the wire colors in their order and match them up match. There are 2 big connectors on this ECM. I'm putting up all the diagrams for both connectors. You'll have to get a real good flashlight so you can see the correct wire colors.
The 1st picture is the 2 connectors,
the next 3 are connector 1. Its a blue color
and the last 3 are connector 2 is clear looking.
You're going to have to find it and check the power inputs and grounds to it.
Connector 1 (blue) has the main power feed for the ECM. The Orange wire (Pin 20)
The ignition switch power is Pin 19 Purple wire. Both these should have battery voltage.
The ECM ground is on Connector 1 Pin 16 Black/White wire.
All the 5 volt refs are on Connector 2.

How long has the pedal been unresponsive? Was that from the beginning of all this?

Okay. For trying to keep a good ground, using an Alligator clip helps a ton. You really need to keep the ground (the black meter lead) as solid as possible. Because with the meter set on a voltage scale, and not having a good ground, the meter is just going to read 0 or 0.01. So have a good ground is essential. Plus it makes it easier to test when you don't have to hold on ground lead all the time.
If you plan on doing repairs yourself in the future (which i recommend you do, because you will save yourself a lot of money). You should invest in a pair on meter leads that have clips on them that can be removed if needed. I recommend (https://www.aeswave.com/)
This is where I get all my leads for my meters and lab scopes. They High quality leads that they build for you. The prices are good too for the quality you get.
This is what you want- (https://www.aeswave.com/Automotive-Test-Lead-Kit-p9121.html)

Now onto the car. The ECM connectors have covers on them that can be removed for testing. Pins 19 and 20 look okay. Pin 66 is not good. With the key on you should have at least 0.4-0.5volts at closed throttle. "There's your problem" if you have the black lead correctly grounded. The code sets because of a voltage signal lower than 0.1v.
Pin 16 Connector 1 is the ECM Ground pin. Again if you have the meter leads grounded correctly, that reading is ok, its reading -0.02 because the meter leads are reversed. The other way it would be reading 0.02, so that's okay. Pin 66 is our issue here,

Are you getting that same reading at the TPS itself on the dark blue wire?
If not there's a broken wire somewhere.
If you go to the TPS and get a reading around 0.4-0.6, somewhere around there. You'll need to follow that wire, start taking it out of the harness and follow it to each connector. Take each connector you find along the way apart and check it for corrosion or breaks.

You need to carefully examine that wire as far as you can.
Eventually you will trace it back to the firewall. Use a really good flashlight and inspect that thing up and down.
Check for the same reading your getting at the TPS, at each connector that wire goes through,
You're getting close now. From what you're telling me, the ECM is not receiving the TPS signal.

And to your questions, no you should not being seeing 12v on that Dark Blue signal wire at the ECM (Pin 66 Connector 2).

I don't think you're going to erase anything by disconnecting the ECM bulk connector. You just have to have the key off before you do it. I have never lost any ECM programming, ie- Fuel Mapping, Shift Points. from disconnecting an ECM connector.
But you wont have any power to ECM for testing if you do disconnect it, usually I only disconnect the ECM plug to remove that cover and plug it back in so I can back probe the connector for testing.

In Honda's service manuals, they have you disconnect the ECM connector to test circuits. So i would say no to that question.

(Updated)
Try this too please, Key Off
Unplug the TPS and back probe the dark blue wire,
Set your meter on Ohms (Resistance testing)
And touch the back probed dark blue wire with the red lead and black lead to battery negative and tell me what you get for an Ohms reading.
This is a short to Ground test.

Here's some extra guides just to help further your knowledge:

https://www.2carpros.com/articles/how-to-use-a-voltmeter

https://www.2carpros.com/articles/how-to-check-wiring

https://www.2carpros.com/articles/throttle-actuator-service

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

Did you find anything yet? Did you try the resistance test to ground? I checked around for information on this.

Apparently the harness can rub threw on the A/C accumulator (which is the round looking can in series on one of the AC lines), and also around the area where the ECM is due to vibrations. So also check inside near the ECM as well. Anywhere the harness has a contact point with the frame or engine block.

You're absolutely certain you had no reading at all at the ECM on pin 66 with the key On? While having a reading at the sensor itself?
Just so we're clear that the Signal is not making it to the ECM from the TPS.

With you opening up the throttle and just getting the air rushing in noise, (which I have had before as well). Seems that the ECM doesn't know you're opening the throttle, therefore not adding any fuel.

The truck I had that was doing this had a bad ECM, the throttle plates would open with the cable from the pedal, air would rush in, but the ECM would not increase the injector pulse width, so no added fuel.
I'm still digging for more information as well.