They may have not migrated due to you feeding them, or they may have stayed anyway. However, it's way too late to stop feeding them now. You should continue to feed them now, as they are dependent on you, for the winter. Come spring, start gradually feeding less and wean them off altogether by late spring. It is never a good idea to feed wild animals. It is not good for them nutritionally, and it acclimates them to people, which can cause conflicts, in which the animal is always the loser. Best to enjoy them from a distance, being wild and doing what comes naturally. Regarding the possibly domestic duck, I'd keep an eye on it, if it flies well, and seems to be part of the flock, leave it be. Meanwhile, check with the neighbors to see if anybody's lost one.