The way I understand it: "nede" is when something is static/stationary (the duck is already down on the ground) and "ned" indicates movement. So you'd use "ned" if the duck were flying down to the ground.
Yes. The difference would always be clear from context. For instance, you can't go 'up' a 'bakke' if it's flat. 'bakke' would refer to the side of the hill, or a slope/incline, or simply to the ground.