Have you tested resistance on the ground wire route path on the car from the TPS connector to the ECU connector?

There is a splice in the path that can be trouble and looked at later

In ensuring that it is squared away you may get away from your 0.70 volts return signal to the ECU reading and come in line with 0.60 and have that variable make more sense as a under control item before swapping TPSs

If it falls in range there is no need to replace the TPS

On the IAC some have applied heat from the inside throat area on the IAC bolts area

 

Parker - no, I haven't tested resistance in that ground wire path so will do so. But I don't know where the ECU and its connector are located, obliged for directions.

 

The ECU on all models is fwd of the right front door hinge under a small plastic panel on the side

There should be a attached ground strap

Use a good meter powered by a 9 volt battery and not a small pocket size meter that doesn't have enough horsepower to read resistance accurately

This is an image for a same right hand steering model

 

Is that the inertia reset switch I see there?

 

Yes , it's going to be in the same as the ECU fwd of the right door hinge on a left or right hand steering model

You know how fun that was in rewiring the ECU harness

I had the large RS3 through connector below that ( not seen ) damaged from the previous owners wreck and some high school got lost electrically putting the car back together

 

Success! My accelerator pedal is back to normal!
I put the original throttle body back exactly as it was, with the sole exception that I exchanged the idle air bypass valve for another one (eBay, from a used body nominally off a running car). And bingo, all is normal again and it seems to be a stable fix. So you are all heroes for helping me through this. Apparently even though it seems to defy the laws of physics, that valve was messing up the vacuum forces on the butterfly to hold it quite strongly in the idle position.
I will still chase down the ground strap measurement as suggested, given the voltage oddity, and also now that I have its circuit diagram I will set up with a 5 volt supply on the bench and see how the original valve works. I noticed that with the complete throttle body on the bench the valve seemed to be closed more tightly than the one on my eBay unit, so perhaps it had walked itself to an improper setting and couldn't reset. But the main thing is that my car is running normally again. Thanks again.

 

I remember someone left out the IAC gasket and that may explain in your case some sort of butterfly interference as the poppet is down in the close to closed step position

I haven't seen mine in a while to see how close to 2 items are to each other

Vee's suggestion of the TPS sensor new value eventually being learned by the ECU makes sense and if what you have is working then that suggest to not adjusting the TPS mounting

The 4 wires to the IAC connector may not be 5 volts and the breakout sheet page before the wiring diagram page will specify the values

These are command or muscle wires vs. a sensor value wire requiring a very controlled reference voltage