That's the thing, she's an indoor cat, we have no idea where the flea came from

Point 1 has to be chosen when the cat is young. Forcing an outside cat to suddenly only be inside often doesn't work.

I adopted a 7-year-old cat from the shelter, and after a week of having to be inside all the time, he got more and more frustrated. After a week and a half, he escaped during the night. In the morning, while I was panicking, he came strolling in as if nothing was wrong.

Since he apparently comes back, I allowed him outside from then on. Since that moment, his behaviour inside has improved a lot. No more random play attacks on my ankles and hands, and generally much calmer.

He has also come back home with mice several times. He always eats them. So I think he is very used to living outside. Maybe been a stray, or a farm cat.

Forcing him to be inside would feel cruel.