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.