If you turn the cylinder to where the pistons are all the way up, is there still the fear of the valve dropping to where the head would need to come off? If you stuffed string in as it nears tdc and then turned it to tdc for each cylinder, wouldn't the strings along with the pistons coming up keep the valves from falling much at all?

We have been there and done that...
Good spark plug pics to compare with here...
Spark Plug Horror! Look!

We did the valve stem seals and it cured this problem instantly.... smog is another issue, but definatly start with these oil seals..
tech article here...
http://www.peachparts.com/Wikka/M103ValveSeals

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2009 Mercedes ML320 Bluetec