Yes, bed liner is the stuff to use!.I would use it too if I could I get it done for a reasonable price.
Mercedes Were NEVER smooth underneath. It's more original looking if the surface underneath has a rough rippled appearance.
Regarding the cost of a new bumper,have you priced a quality rechroming yet?
$1200 is not unusual for a bumper and it may cost even more in your location because of government environmental constraints on chrome shops..