Maybe not the ditch, Polly - ....although it is a 13 year old Merc, and in the last 3 yrs its been.on a hoist more than.its been on the road.
It can't do what it once did....and Merc parts have always been damned expensive & hard to get & sort out.
But I would be having a stern word to the dealer and his mechanics, because the service department just can't seem to get things right.
Personally, as a starting point, I'd be moving the service manager on.....