I don't how anyone could adopt a previously feral cat and not know it. If you are going to make the decision to adopt a pet, would you not go back maybe several times and visit the cat, so to avoid having to bring it back ? Also, when you adopt a pet, the expectation is that you are in it for the long-haul.You accept them warts and all. If they have any medical problems, this is just how it goes. If you truly care about the pet you have adopted, you just suck it up. There are no guarantees.
As far as the statement that it is much more humane to end an animal's life than leave it in a shelter for a long period of time. Well tell that to the animal in question. Do you think that they would want to die simply because they have become inconvenient ?
Using the term "putting an animal to sleep", is a dishonest euphemism for the callous killing of a living being. Have you ever seen an animal euthanized? Well it is not very pleasant. I've read descriptions of dogs who struggle furiously when dragged to the killing room. They know what it is! Some of the dogs are dragged while urinating and defecating, they are so frightened. It doesn't do them any good as they are powerless and they did anyway. I've read descriptions of how cats struggle in vain in the gas box. Read it and then say again that your industry "puts them to sleep". Why do they fight? Because they want to live. Just like the rest of us.