I'm so sorry, AJane. It's tough to feel so helpless and it's especially hard when the animal is relatively young. I have no wisdom for you on the subject. Just trust yourself to know when it's time to keep trying to find a solution and when it's time to let go.
Re: the feral cat. We had a tom cat (okay, we fed a tom cat - I don't think you ever actually OWN a tom) years ago and he'd show up with the most appalling abscesses and we'd think, "Well, that's it. NO way he can survive this" but he did. It's amazing what their little immune systems can withstand. That cat lived for maybe 15 years, I think. No one could believe it. Oldest tom ever.