An aside on your last point: In the D gear, a lot of auto transmissions will be set up to allow for the wheels to spin somewhat independently of the engine (in any gear) when you aren't accelerating, which accounts a lot for how well an automatic will coast when you release the throttle, even if you're not going fast enough for top gear (especially apparent with older four speed boxes).
Using the numbered gear options will enable clutches/bands that provide more engine braking.
Using the numbered gear options will enable clutches/bands that provide more engine braking.