So there is a mismatch on the diagnosis. I suggest getting more neutral information from the franchise mechanic about why this mismatch could occur. You could ask the franchise mechanic how he diagnosed the injectors issues and his opinion about how someone else would miss it. Interesting to know if it was diagnosed with a computer or other means. No idea if a computer will show or an analysis of data will show injector wear. Others might like to comment on that. You could then ask the dealers "specialist" what equipment he used to diagnose the issue and talk to him about the mismatch.
You could ask the franchise mechanic his opinion about replacing the NOX sensor. Perhaps he is well aware if it has no impact on the problem but if it is clearly an issue for other critical functions like emission control and it is just something he picked up at the same time.
You can also ask the franchise mechanic his opinion about 'adjusting the idle'. No idea in this particular case but in general idle adjustments can mask a multitude of problems.
The obvious way to go after all that discussion is get a second diagnosis from another franchise level or similarly equipped workshop and see if it matches the first. The dealer will find it hard to argue with two opinions.
It looks like the CAB is suggesting writing a letter formally asking the dealer to sort it out fully based on that other diagnosis. If CAB are suggesting it then maybe that is the right thing to do, but you should ask them about the next step if that comes back negative which is probably will.
If you decide to let the dealer's guy attempt to resolve the issue personally I would take it to the franchise after that to see what they think of the resolution. If the resolution is just masking the problem or worse that might be the time to pay for the required repair and file a case with the DT.