Where did you hear that nugget about the OM651 timing chains from? sounds like utter bolleaux to me and certainly never mentioned in any M-B training material. Given the amount of clutter at the front of the OM651 & OM654 engines, putting the timing chain there as well would make the design of the coolant galleries problematic at best.
Yes, going to a simplex chain is emissions related - but they're not really that bad of a job to replace, not compared to a lot of BMW engines - also at the rear but with far more room between the bumper cover and the front of the engine.
Wet belts are fine in a controlled environment, once 'out in the wild', there's far too many variables that will affect longevity for them to be viable.
From mercedes own literature. Last paragraph.

The whole document is here.