Consider yourself lucky. I had a similar situation on my old Ford farm truck when I built a motor for it. The machine shop used the wrong retainers, and I couldn't figure out what the problem was. I finally figured it out when it dropped a valve.
It sucked part of the valve head to an intake port on the opposite bank, I pulled both heads and fixed the problem including replacing a piston. I was just lucky that it didn't get the cylinder wall.
I would expect that many engines have turned into scrap iron by machine shops that didn't know what they're doing. The problem I described was on a Ford. If it's hard to find somebody that knows what they're doing with Ford heads, just think how hard it is to find someone who knows about a Benz.
Consider yourself lucky, and put it back on the road.
'84 Euro 240D, 516K miles
'88 300E 5 Speed
'81 300D Daughter's Car
Over 800,000 miles in