Well, just damn. You don’t have a lot of options, do you. That sounds grim. Sorry.
If I were in your shoes, I would trap, spay, and release her.
When we had our first two cats, if we left the front door of our condo open (had a screen door, too), a neighbor’s cat would
sit outside the door, a tiny, tiny Siamese. She was an adult, just very small. She just sat there, staring in, not saying a word.
Muffin, our male would go berserk and attack our female, Tiger. She was a little *itch, and she held a grudge against Muffin
for two weeks at a time.
We stopped leaving the door open, and we opened the sliding glass door to our itty bitty fenced-in patio instead. The little
interloper couldn’t get over that fence. She was too tiny.
Awwww...I love kitty stories.