OK just read it all and to my surprise NOBODY has reported back that the idle Air Control Valve was checked to see if it's rest position at normal temps was within spec or anywhere near spec. IIRC it shouild be about 0.04" or 1 mm.
Just saying it 'works' when called on by the ICU/ECU to regulate idle isn't enough.
The base idle is set to regulate around a IAC valve position that is governed by a spring and bi-metallic element. This position fluctuates with age, and it opens wider when old, letting more air through the bypass loop.
The ICU can regulate around an old IAC to a point, but if the initial position is too wide open, the idle will surge at first then the ICU will control it down, and compensate but the spring will over compensate and raise the idle again. This may go on until there's a default reached where the control ceases.

The initial spring position of the IAC valve can be adjusted by clearing out the potting on the external adjustment screw, and the valve returned back to the spec position, but it CANNOT compesnsate for the error from the temperature dependant bi-metqalic element being worn, and will give erroneous position as temperatures change, like from cold start to warm-up, and as the IAC circuit struggles to recover the correct idle the speed will fluctuate or stay high.

My impression - early in the morning and without searching out or re-reading all your threads, is: If this was in my driveway, I'd try to find out where in the control range the idle valve is.

Let me try to explain, from a theory perspective -- not experience here; I haven't needed to do this, and don't really expect I ever will, unless by some chance someone with your symptoms shows up at my door:

To oversimplify it, the idle speed regulation occurs in a feedback loop. The RPM signal from the crank sensor is passed by the ICU to the ECU which uses it to control the "on time" or duty cycle to the spring-loaded idle valve.

To be effective, the idle valve must be able to pass some air around the throttle plate in all temperature and engine conditions. How much air, is determined by that duty cycle. If it was instructed to pass no air -- fully close itself -- the feedback loop would be at its limit, and useless. Likewise, if it got enough on-signal, it would fully open, and again, break the loop.

My approach to isolating the nature of the problem would be to find out where in that operating range -- between fully closed and fully open -- the idle valve was working. Simply pinching the hose is the quick diagnosis for high idle. If it starts to stall, you know the idle valve is what is keeping it running.

A dwell meter can tell you where in its range it is. The idea is to find out if the air (to create fast idle) is being ordered by the brain or involuntarily provided by another leak. Or is the brain sending the right signal, but the muscle isn't moving...

I would assume your shop has already determined the computer is telling the idle valve to open too far, which means you could stall your car if you pinched the idle bypass hose, and maybe bring the idle down to normal by disconnecting the idle valve's electrical connector. With no signal, the valve is open a "limp home" amount by design.

If the problem were related to a faulty signal from the crank sensor, as would result from any mechanical wear (I would not think loose bearings - but might consider an incorrectly installed flex plate or loose tone ring damaged during a transmission service) that faulty signal would be easily identifiable using a scope on the ICU signal to the ECU (ICU pin 17).

I have to think how many times I've learned things only to remind myself: None of us, including pros, were born knowing this stuff. It is an honest person who can admit that and deduct that learning from the food-on-the-table income stream. You owe them a debt of gratitude for that honesty, nothing more. Repay with your own honesty.


--
Art Benstein near Baltimore

A Stranger is just a friend you haven't met yet.

I have no doubt they are treating you fairly by your description (ok, maybe a little doubt), but I also have no doubt that one of three things are happening here:

You are not understanding and passing on what they are explaining to you very well

They are giving you multiple unconnected bits of information and you are piecing them together in an order that makes sense to you but does not actual make sense.

The shop is completely out of ideas but unwilling to admit that they had just thrown parts at it in the past are going to do so again.

Since I always want to give what sounds like an honest shop the benefit of the doubt, lets try to unravel what they might have been talking about.

We all know that they suggested the tone wheel on the flex plate could be bad and it is at the back of the engine and hooked up to the transmission more or less. There is no bearing associated with this part that I can think of. It is almost impossible for this to have an intermittent problem.

There are no sensors on a Volvo 240 that can be adjusted.

There are no adjustments you can make to the computer for the sensors. I forget what year your car is but they can reset the "trim" values back to default by removing power to the the ECU for a few seconds...but that usually makes the car run worse initially and get better over a few miles of driving.

The only sensor that even remotely has to do with NoX is the O2 sensor in the exhaust which even the most expensive are $140-160 with less than a half hour shop labor to replace. Sure the Air Mass Meter, temp sensor and knock sensor all contribute to controlling the levels of of NoX in your exhaust, but only one really measures anything to do with NoX.

Most of all, I beg you stop throwing money at this (which you have already decided to do). With combination of your car's problem, your shop, and your understanding of the issues, this problem is unlikely to ever be resolved no matter how much money you do throw at it. This is not intended as an insult to anyone...just a reality check.