My husband and I maintain a feral colony and do our bit for homeless kitties in our town..we see some stuff we'd rather not! But I never seen such a bad case as this kitty. We've druggies nearby and sometimes they come up with creative jokes (how they can get the ferals I've no idea, they should come help me trap).
Traps can damage a kitty but not as badly as you would think besides you'll be taking the kitty to the vet and lucky you have a barn, you can sanctuary her (my dream, we have a small house, small yard and no fence so we rescue as much as possible but we're limited, hopefully when we move we'll get a larger place) I use Tru-Catch traps rather than haveaheart because they are lighter and more manageable but it is up to you. Contact your shelter/rescue group they could help you for sure..
Best luck