I think your GC will eventually accept your Mallards it just takes time and with her losing her mate that makes it a little hard to accept any new potential mates. Even if your Mallards lived with the chickens for two years they are still new to your GC duck.
If you want to make this work trying keeping them where they can see each other but no one can touch, temporary pens would work. Just tossing a new drake or duck in with an established ducks always causes trouble. Once they have lived side by side for a while try letting them all out together...