If aviation authorities have mandated inspections of all airplanes with a specific engine model, I suspect they may actually have more merit to claim extraordinary circumstances in this case. This brings it closer to "hidden manufacturing defect", which has been ruled to be extraordinary circumstances. The fact that aviation authorities have mandated the inspection may also be seen as providing the "external" in "unpredictable, unavoidable and external ".
However, you should still ask to see some form of evidence of these mandated inspections before you buy their explanation.
If you give up on trying to argue with Norwegian, you could always try to contact a claim agency specializing in EC261 claims and present your case to them. If they don't think you have a good case they probably will not pursue it, but there's not much harm in trying and if they do agree to pursue it you'll get most of the compensation with a minimum of effort.