I remember having gone back to the dealer when I bought my 1989 W124 bought used in 1991 with exactly the same comment re: excessive roll.
I was told it was normal.
Since then I simply forgot about it.
Now at closed to 400K, it still rolls (backward or forward) with no apparent bad consequence.
parking brake???? what's that?.
I don't think I've used it more than 3 times in the last 16 years.