"belt dressing", comes in a spray can from auto parts stores.
No, an accessory belt is not considered a serpentine belt. However, a serpentine belt is considered a specific type of accessory belt.
The belt is probably glazed. Clean all the pulleys and replace the belt. Do not use a belt conditioner on a serpentine style belt.
Never use belt dressing on a serpentine belt. Clean the dressing off and recheck the pulleys.
Diagram is on the engine by the belt. You may have to clean off oily residue to see it...
The serpentine belt would be the same as the accessory drive belt.
Serpentine Belt Diagram ML320
Loosen the belt tensioner. Make sure that the belt can be removed easily. Replace the belt with a new one. Check that belt is clean before installing. Clean the pulleys if you have to. Tighten the belt tensioner to set proper tension.
Serpentine belt, no. Timing belt, good chance yes.
If the alternator belt is the same belt as the other accessories, yes.
That I know of, there is NO serpentine belt, unless you are talking about the the Timing Belt (hardly serpentine, but anyway), it has a belt tensioner, not automatic, though.
Loosen the tensioner pulley. Slide the tensioner pulley to the right. The serpentine belt will come loose. Remove the serpentine belt. Reverse the process to install the new serpentine belt.
check on your fan shroud there should be a diagram on there for serpentine belt, etc. Clean your shroud off as it may be dirty or oily and you may not see it immediately.