The answer to this HAS to be the braking answer, as in all cars aggressive braking will result in inefficient use of the cars momentum.
Driving above 50 is a nonsense answer, my s-class for example is most efficient at around 70mph, 50mph is too slow for top gear hence the car changes down.
If you do a motorway drive it doesnt matter on your braking as you do so little does it? We dont know if agressive means simply quick braking or braking quickly such that you can an overshoot of speed retardation so you end up slower than you intended to be and hence have to accelerate even more after the braking.
Aero drag is the main force a car has to overcome, speed gives this force by result of doubling the speed = 4 times the drag. The engine burns fuel to give this.
There is something odd with your merc if you get less rpm at 50mph. The fact its not in its top gear means nothing regarding the engine efficiency when you might be sat at a better BSFC point at 50mph in 4th than 5th. I guess the main reason is that you dont get direct lock in your auto box in anything other than top gear?
You answer kind of ignores physics and facts using your own opinions, but I guess we're getting used to that from Jez.