@xulfiqar Does the Mercedes transmission has a sprag clutch between each of the gears? I admit there is not enough data for Mercedes in hypermiling forums; most of them drive hybrids, or 3 or 4 cylinder econoboxes.
From what I have read and experienced, most cars have a sprag clutch only between first and second; that's why the the transmission freewheels in first. In higher gears, the transmission doesn't freewheel, but it is locked down to the engine. In a Honda, there is no sprag clutch between first and second; at least I assume it doesn't, because it engine brakes even in first. I have also read somewhere that older Chevy transmissions freewheeled in first and second (in a three speed). Maybe they also have sprag clutches between each gear?
As for flat towing in owner manuals, I have pulled four Honda user manuals, one '01 Impala manual, and one '09 Corolla manual, from the internet. All the Hondas ('96, '03, '06 Civics, and '08 Civic hybrid), allowed flat towing below 35 mph for 50 miles. The Impala manual simply told to call towing services; the Corolla manual explicitly stated the use of a towing bed.
BTW, if your car really does freewheel like that, then all I can say is that that is one well engineered car.