I so understand. It is never an easy thing and I do not think you really ever get used to it. Some production people can just so it whenever necessary but it is much harder when you love them and see them as pets. *hugs* talk to the vet or call Penn State and ask them because I am sure they would do the deed and then perform the tests.
I am sorry if I sounded mean or heartless as I really am not either. If you were close by I would come do it for you and be crying the entire time. I really have to talk to myself and remind my heart that it is the best thing for the bird and the right thing to do. The hardest part is that you never can be sure...you cannot see inside them and see disease. All we can do is work with the information we have and make our decisions. As much as I love each of my feathered babies, I would sacrifice any one of them to protect the rest and if one was ill like yours, u would sacrifice that one to make sure I knew what I was dealing with.
She could have some rampant infection that is unique to her and not contagious but can you take that near impossible chance? Disease mutates and you can never be sure who or when it will affect someone else. I do not envy your choice. Only you can decide when the right time to act is. You can try giving her a baby aspirin every day to help with any pain and for inflammation
And make sure you become a nuisance to yourself with biosecurity whiles is alive and in your house. Hand sanitize and different shoes and clothes...set aside clothing just for wearing around her...
Keep us posted in your decision and your girl.