Usually when they're this sick, there isn't much time left. If you were to make an attempt, you'll need to do some force feeding, but it truly is a judgement call as sometimes adding more stress when they're soon to pass feels wrong. We use raw honey, heavy cream, and electrolytes in these situations. Just be careful not to feed more than they could naturally eat.
In terms of the body of a deceased bird, we have always buried them, roughly 18 inches down. We have a ton of predators and scavengers in our area and haven't had any issue. Sometimes we put it in our compost mound, buried about halfway down so it won't inadvertently be exhumed when we fluff up the pile. This way, the duck lives on through any subsequent garden or flower bed. A lot of folks around here will feed their dead chickens to pigs or dogs, but i don't think that's wise, especially if it died of illness. Also, it can trigger predator behavior and create needless risk in the future. When the ground is frozen, we burn the bodies, if you do this, just be careful to have plenty of wood beneath it because it needs a long lasting, hot fire to work properly.
Sorry about your experience, and for your duck. It is so tough to see them in pain!
