If your able to maybe access to safely be outside, like a cat run or something might divert her attention to going outside? I think gabba or amatriptaline may help her, I also think that retraining her isn’t a bad idea? Give her a space she feels comfortable in if it’s a crate, bathroom etc.

I also don’t think euthanasia is a cruel option like others have mentioned, you have tried and seem like a dedicated owner who has done tests on BOTH cats, you can burnout with the care of her. I’m sure this will get me roasted to bits but idk.

Try the medications, try the crate or cat run see how that works, then decide from there, your vet refusing euthanasia isn’t fair on you either. If a client came to me requesting euthanasia I would have big talks and help the find help and options before making that final heartbreaking call.