I would have one serious conversation with the Mercedes dealer followed quickly by another very serious conversation with a good attorney. Know the outcome you want: Mercedes dealer buys car back and reimburses repairs, new engine, whatever. Listen to the attorney and be aggressive.

Good luck.

__________________
Mike

2005 e53 X5 4.8is 169k miles
2018 f90 M5
2018 Volvo XC60