To be certain what it is the cause the whole system needs to be analyzed .
OK start with the DEF fluid what is it to the specified gravity? Take a test sample.
Next step, test the dosing system using a bi directional scanner ,will the pump deliver 5000HPa under test and in practice deliver 3800 HPa when working?
And the dosing valve -take it out, look at it furred up, , test it & does it deliver the specified amount delivered into a burette in cc's.
OK next step is the exhaust system flowing properly , might the Cat be partially blocked up ? Check for stored ash & soot on the scan tool
Read off EBP and then differential pressures across the SCR cat. Do they meet spec?
What are they getting in live a data figures if they don't know ?
On a road test is the system actually trying to reduce NOx , is it dead , or are the reduction across the SCR cat more than 50% NOx reduction minimum or bests 75% plus.
If it goes lower than 50% it will flag a code.
These are the basic tests that need to be done & recorded by scanner print out and presented you as the customer to approve fixes. NOT GUESS .
This sounds from your narrative like Mexican shops on the Baja that don't see these systems too often, unless its US/Canadian Gringos visiting the peninsular with MB products, like Sprinters .
As back story MB is traditionally "tight" with tech info unless you are dealer and they don't like sharing info with Independents.. Get the picture?
This is in direct opposition from Ford USA that conducted FOC training for independent techs nationwide in 2010 .
Since F Series truck has the same Bosch system as all MB's it was easy to use the tutorial in the early days to analyze and fix these systems without damaging Ford product confidence .
Tuercas viejas