Sorry, but this last bit was just too much.

The amount of energy lost to friction in the freewheel clutch is so tiny as to be meaningless. In any case it's much less than the amount of energy lost to friction in a chain drive.

If anyone doubts this, I suggest you set up a freewheel bike (single speed or derailleur doesn't matter) and a fixed gear bike as well as possible. Bring both wheels to the same speed, and let go of the cranks. Does anyone want to bet a couple of beers which coasts to a stop first?

This last claim is why I responded, but the first part about momentum in the legs is equally flawed, maybe more so. Our legs are not well oiled machines and it takes considerable energy to work them, even without producing any power. Riders of fixed wheel bike know this because they need to work even on descents, not to power the bike as much as to help keep the cranks turning and not be bouncing around as they turn.

We run into parallel issues which may be instructive in the auto world. Those looking to maximize fuel economy will either clutch or shift to neutral and "coast" down hills. BITD Saabs used to have an overrunning clutch so they could "freewheel" down hills and save fuel. The problem with freewheel descents is that the engine braking is important in that it's drag helps the brakes and prevents brake fade, which is why the practice is reserved for fuel economy contests.