Was the water pump replaced because it leaked? As cooling systems age, the components weaken and rubber and seals fail. The leak will occur at the weakest link, and the rest of the system may appear ok. When the leaking component is replaced, though, it is not unusual for a new leak to appear at the next weakest link.
Unless the new coolant was in the system for a long time, it is unlikely it would have in any way contributed to system failure. Old coolant from MB or anyone else is far more likely to cause problems than any decent new coolant.