Interesting.
If you are at 2500 rpm and let go of the accelerator, then a fuel must be being used otherwise the engine will stall wouldn't it. No fuel, no spark, no nothing.
In Eco you're actually coasting, and so engine braking isn't slowing you down so you can go further when off the accelerator and 650 RPM is using less fuel than 2500 RPM.
I'll have to look into this fuel supply cut off when in normal mode.I didn't know about that.
If you are in auto mode and lift of the throttle, if the revs get to low it changes down a gear automatically, no stalling. It may use fuel to blip the revs for the
lower gear if you're in sport mode.
Edit sorry didn't read your whole message
Fuel being used at idle coasting out of gear over a distance is going to use fuel.
Coasting in gear is going to use no fuel, but admittedly you may slow down more.
So what's going to use more fuel, going from say accelerating from 55 to 70mph having used no engine braking but fuel on idle and then fuel to blip the throttle up to the right speed.
Or
Accelerating from 50mph to 70mph having used engine braking but no fuel on idle and no need to blip the throttle.
On a safety note, in all my driving lessons i was always taught not to press the clutch in an emergency stop until i was already braking. Those small moment of engine braking when moving from accelerator to brake pedal may make all the difference from an accident to avoiding one.