How to squeeze the calipers
Most of the manuals suggest that when replacing the pads, you squeeze the piston on the caliper back with a pair of slipjoint pliers. This is hard to do, and if you can't get the piston squeezed all the way back the pads won't fit back on the rotor disk.
What to do?
USE A SIMPLE C-CLAMP! A common 4 inch screw up C clamp is the ideal tool, and has enough leverage that you will be able to easily screw the caliper piston back easily. Open up the clamp, put it as close to the center of the piston as you can, and screw it down.
The rotor wil probably come off with an application of WD-40 and a sharp rap with a rubber hammer. If you do not possesss a rubber hammer, put a piece of wood (at least 2 in long or thick) against the rotor and rap it in several places around the rim with a regular hammer, always cushioning the blow with the wood.
Remember those rotors were put on by Helmut Hans Schtreubelmacker, and he was much stronger than you are. After that, they were tightented by repeated coolings and heatings for many many kilometers. (as we know, a Mercedes always thinks in kilometers).
Semibodacious Transmogrifications a Specialty
1990 300D 2.5 Turbo sedan 171K (Rudolf)
1985 300D Turbo TD Wagon 219K (Remuda)
"Time flies like and arrow, yet fruit flies like a banana"