Your suspician that the conversion was not done correct is probably accurate, but the reason that the low side fitting was attached to the high side is this: mercedes doesn't have different size schrader valves on the low and high sides like all the american cars out there do. If you buy the new fittings from napa or anywhere other than a mercedes dealer, the high side fitting isn't going to fit because the schrader valve is too big. To remedy this, buy the correct adaptor fittings from a mercedes dealer. They ran me $10 each, but they are also of much higher quality than I've found anywhere else with the added bonus that they fit. I checked fastlane for the part and didn't find it...

The other part that should have been changed was the accumulator/drier. Another thing that really could have been done wrong was if the PO mixed the two refrigerants or different kinds of oil. These things could potentially be bad. If suspected, it may be a good reason to do a system flush and/or drain and refill of the compressor oil.