I am so sorry about your dilemma. We are in the same boat, have tried everything including onsite visits with a behaviorist and putting the new kitty on anti depressants per the vets advice. Nothing is working and it's maddening to have them seemingly get along with a baby gate between them, but once the gate is removed the chase is on once again. Have never had these problems before and have had a lot of cats in the past. I'm sorry I don't have any meaningful advice for you, but wanted you to know that you are not alone, and it sounds like you have tried everything and are a great cat parent!Yeah I can't sit three cars in a 600 square foot apartment.