Should I get my duck Walter an amputation
It's a personal decision. I don't envy you.
Most ducks have a genuine joy in living, and they are not quitters. I would not give up on Walter. Whether you decide on some kind of device to help him get around (there are duck equivalents of walkers and wheelchairs) or go with amputation and perhaps a prostethic (look up Buttercup the Duck), Walter has a good chance at a lovely life.
Even Lemon, who has a bunch of physical handicaps, has led a happy life.
I wouldn't give up. As long as your baby doesn't look super uncomfortable, I do not see why the amputation would be a problem.
We have a special needs rooster, whom has weak legs after getting in an alteracation with my neighbor's rooster (he was my neighbor's, but she gave him to us to separate the two). He is unable to walk long distances, but he is still his feisty self, and you can tell he enjoys life. It all depends on how much your baby wants to live.
I don't want to worry you - - - I have heard that waterfowl surgery can be tricky due to their sensitivity to anesthesia. Did the vet go over that with you? Maybe he's figured out how to avoid whatever it is that has caused problems for others.