Oh my, a staple. I'm so glad you took her in and they found it. I think it must not be uncommon for ducks to ingest metal because my vet is frequently taking radiographs of my ducks to look for screws or similar items when they aren't feeling well. Even when we don't find something my vet will do a blood test for heavy metal poisoning just in case the metal item dissolved.
I'm constantly after my partner to be careful with his projects, not to drop nails or bolts... But of course mistakes happen. And ducks pick up and swallow things so quickly.
I know you are likely very scared of your cayuga having surgery after the way Samarra just passed away. Hopefully your cayuga would be having her surgery at a different clinic with more experience with waterfowl? My ducks have had anesthesia multiple times and always recovered without an issue. The clinic I use is very experienced with birds.
I completely understand how you feel. The emotional rollercoaster of having ducks has also been unreal for me. I adore and cherish them, and I am heartbroken as well. I have had to double my work hours to keep up with my ducks' medical bills. I'm sorry I can't offer anything besides that I relate and understand how you feel and what you are going through.