Change mechanic. Go to an LR capable guy/girl. The condition of the injectors won't be known unless put in a test rig.
Most mechanics will have a look at the pattern on the piston crown for "visual" check before deciding to take further action.
Since the visual inspection requires the head to be removed or a "boroscope" (internal camera) and these haven't been mentioned,
I'm assuming neither have been done.
As earlier posts also state - you would be the unluckiest person in the world to lose 4 out of 5 injectors. Of course bad fuel could do it - but then your mechani would certainly be telling you(not suggesting) to drain and clean the fuel system completely.
Let us know what you decide to do.

