I don't think that eye is comming. Usually when a "disabled" animal has a major defect and minor defect they usually end up being the same or very simmilar, especially with eyes.
I love reading this thread, it is soo touching. I've never had an animal born with defects, but many accidents have happened on the farm. Cat got ran over (lived after expensive surgeries, but totally worth it). He's still alive, and my favorite cat. My dog was ran over by farming equipment, now she has three legs, but can still run faster than everyone lol.
I do like the idea of another duck living with him/her. When the duckling is an adult or getting there, you should introduce a younger one with it. The younger one will bond with it, and care for it while your not. This ofcourse is if they are kept separate from other ducks. It was a good idea, a very good one. I don't really know my ducks, so this might not be a good suggestion if ducks have a strict or aggressive pecking order.
Have fun!