I don't have a problem with just draining out the old stuff (completely as reasonably possible) and refilling it. The mechanic should have drained the block also. In my opinion that's the only thing he did wrong.
What I believe you have done has left mostly water in it.
Is there a dealer nearby that you can obtain the MB coolant? I'd get 2 gallons of it.
It is difficult to accesss the block drain without the car being on a hoist.
I'd drain it, put in 1 qt of straight MB coolant (undiluted) then use 50:50 MB coolant/water mix, because there is likely a rather weak mix in there right now, the 1 qt will make up for the existing weak mix that won't drain out.