Did you say YOU checked the chain when "new"? Or did you have an independent tech check the chain? Many times there were no obvious signs of problems with the chain itself, but more a problem on the camshaft sprocket whereby the teeth become worn and/or broken. Mercedes service literature says to replace the timing chain sprocket and tensioner with updated parts if you exceed 3 degrees stretch when measured at the camshaft to the crankshaft. Or if you see worn/broken parts as described above. If it infact that many teeth on the camshaft sprocket I would guess you did crash the valvetrain. Usually this does not hurt the pistons but it still could.
99 times out of 100 this problem is caught by a qualified tech before it becomes this big of a problem. Sorry for the lousy experience and bitter feelings you must have on this product. This is a very isolated case.
ASE CERTIFIED MASTER AUTO TECHNICIAN
LEAD TECHNICIAN FOR 14 BAY FACILITY
MERCEDES SPECIALIST 8 YRS
PARTNER IN MERCEDESSHOP.COM