I initially thought it only required the affected seat to be unoccupied but it appears almost any seat belt problem causes non-compliance with the ADR, thus failure of a roadworthy, and thus uninsured because it is unroadworthy if driven in that condition excepting going to the repairer (dealer). I presume it was promptly rectified by installing the missing bolt or whatever caused it? Claiming it was not a warranty item is a bit over the top in my opinion.
Depending on what is causing the engine warning light it could be a problem. An old post about a real event in a land far away. It is always important to get everything in writing, including being fobbed off if that is what is happening.
If you have another dealer within reasonable distance I would investigate their services; while all dealers are expected to do warranty work some are more willing than others because they usually are not paid well to do it by the importer. OTOH many do, being hopeful of getting a few years of service business in return, that is profitable.
