Ducks and geese, like chickens, are flock animals. Your bird needs a companion. If you can't get the other bird, you can give yours a small stuffed animal as a companion along with the mirror. That will help some... It probably misses its friend.
That seems like a lot, but not likely what killed her. Scratch is to chickens what candy is to kids. Give them all they want of it, and they won't eat their nutritious food - the pellets, which are (or should be) balanced nutrients, protein, etc. A little scratch teaches your birds you're the...