1. As others have said I haven't had any yet (TDI with 6sp - 60K miles). Others have 3x that I see
2. When you are accelerating your engine is pushing the car, you use fuel. When you decelerate your car is turning the engine, you use no fuel - fuel is cut off. It is tricky if not impossible to get to the neutral state in between the two for any length of time.
3. You have to experiment to select when to you the different techniques. If you P&G on the flat, coast out of gear as you will coast longer. Same for shallow declines.
EDIT - in this state the engine is idling and it is burning fuel to idle.
For steeper declines you can use in gear coasting to control your speed and enjoy fuel cut off.
Even for relatively shallow declines you can use a higher gear and get the car to turn the engine over to get to fuel cut off.
Or you can shut the engine off on long out of gear coasts but remember your engine also provides breaking assist, power steering etc. which will also go away. I don't use this as I have a turbo.
If you have an instant MPG readout or an SG then you can see the effect of each one.
Good luck!