Remove the fender? I'll have a look at my 560sel this weekend and see what is involved. Just changed the passenger hinge on the 928 Porsche and it was simple. If Porsche didn't make it hard - I can't see why Mercedes would. If you want a real nightmare try to change the valve cover gaskets on the Porsche!!!
On your tensioner - you are talking about the bolt that tightens up the tensioner after ajusting it-- right? If so - take the tensioner off and try to thread the bolt and tighten it into the hole - if you can't draw it tight and it keeps spinning it's wheels - you may have bad threads in the hole the bolt screws into. If that is the case you will have to get the thing re-tapped. It could be that whoever had the car before cross-threaded the bolt and screwed up the threads. That is all I can think of. Anyone else????