With the exception of the older fluid coupling trannies (pre-73), MBs do indeed tend to roll backwards at idle. A result of the fairly high slip torque converter, I think.
Use the parking brake to hold you on slopes like that, then run the engine up before releasing it if you think you will roll backwards very far.
1972 220D ?? miles
1988 300E 200,012
1987 300D Turbo killed 9/25/07, 275,000 miles
1985 Volvo 740 GLE Turobodiesel 218,000
1972 280 SE 4.5 165, 000 - It runs!