Well cars have oil pans for a reason, to keep excess oil from spilling out of the engine. I really can't imagine anything GOING WRONG with even a quart of extra oil. The only time something could go wrong is if you take a turn at 3Gs and a bit of the oil spills out. It MAY catch on fire if it hits a part of the engine that is above motor oil's flash point (450F+- depending on the oil brand). And yes, your engine probably is running cooler and you're probably experiencing slightly better gas mileage and less engine wear. Oil is a lubricant and a coolant. It prevents damage. More oil won't create more wear. It MAY spill out if there's too much, if you hit a turn harder than an F-16 in flight, but I doubt you're doing that, and I doubt the oil pan in a Mercedes engine is so poorly designed as to allow any oil at all to spill out unless it's bubbling over the top.

If you want to go in and hold these people accountable, then do it. Ask them what damage it could have caused. We're all telling you that it didn't and you don't need to worry.