Hello Becca,
sorry to hear the female is giving you a hard time. When I read your thought it could be a neighbor cat that sprays on your stuff, it came in my mind that seeing a neighbor cat could make her so upset that she starts marking.
I started caring for 3 year old "stray" which I took indoors. She was in heat and sprayed. Did drive me crazy. I wanted to wait until she was out of heat to get her spayed, but it lasted for weeks. I had enough when I was sitting in front of my computer, just wanted to bite in an apple, when she jumped on my desk, rubbed her cheeks, turned her but towards my face and sprayed me in my face! Urine dripping over my apple, my hand, my chin... and down.
I took the phone, told the vet I wanted to have her spayed *now* and prayed for her that she would stop marking once spayed. (She did, lucky her!)
Kitty "terror" can be hard to deal with. I have one that tries to force me to feed him by chewing on my orchids. He never chews on them, only before feeding time and only when I can see it. He is a rescue too and did the same with plastic bags, when I got him. Walking away from the plastic bag and the kitty was much easier than walking away from a him when I ruins orchids
Smart kitty, luckily I love him to death...
Anyway, all this OT bla bla is to let you know I sympathize with you... I'm sure, in the end, you will be creative enough to deal with it.
I for instance put a lot of orchids higher on a shelf and blocked it, so that the cats can't jump in between my beloved chids...