Is damage caused to an engine by a Spark Plug an " external influence"?

My knowledge of engines is poor. How do I continue the fight for a warranty claim?

Merc C180
Year Model: 2019
Mileage: 42 000km’s
Diagnosis:
• Vehicle misfiring usually indicates a fuel / combustion concern. For this reason a fuel test was requested.
• The fuel test passed the SANS specifications and therefore indicating that the concern was in the engine itself.
• The Dealer was instructed to remove the cylinder head where it was found that a piece of metal has damaged the bore and cylinder head.
• The metal piece found is similar to that of a spark plug end, however, all the spark plugs on the vehicle are intact, indicating that the spark plug might have been replaced. We have no record of this being done on our system.
• The metal piece and a spark plug is on route to Metallurgist in CT to analyse and provide feedback. The purpose is to determine the composition of the metal piece.

In conclusion:

• No record of spark plug change on this vehicle by any of our dealerships – no claim submitted.
• Metal piece caused damage to the cylinder bore resulting in the need for a complete engine replacement.
• Cost of repair cannot be claimed under Warranty & Maintenance as cause of damage is classified as ‘external influence’.