Nat, I'm jumping into this waayyyy late, and for that I apologize.
Euthanization is not an option. I teach special education, and if I shared the philosophy that every child that I knew that had issues with physical aggression due to disability or a chemical imbalance needed to be DESTROYED, then I guess I wouldn't be accomplishing much in my day to day interactions with them. Period.
I am hesitant to say that medication will be the sole answer here. I believe it may help Ophelia, and even more so if she's placed in a different environment. And removing Ophelia's presence will definitely help Lily. From my experience with medication (in children, mostly), I find that it takes the edge off of the underlying behavior or issue, without entirely eliminating the issue. Now, if you combine the use of medication with a different environment, that may be the answer to everyone's prayers.
I think the most you can hope for at this point is to think of the greater good, in general. Please don't torture yourself with all of the "what-if" questions...there is no clear cut answer to any of them, and you'll be putting yourself through much more heartache and agony than you deserve. I think deep in the recesses of your heart, you know that the greater good for you, Ophelia, and Lily would be to re-home Ophelia. And fortunately, you've been exploring all of ways this can be carried out...
And please go easy on the guilt, if you're able. You tried to give the situation everything you could, and have spent a great deal of effort on trying to make this managable for everyone...there are many people who could not claim this to be true for their own situation. So now, you must act out of love, and think of what is best for Lily, and Ophelia.
Personally, I would opt for a combination of the medication (I'm interested to see how Ophelia will do on the Buspar), and a quiet, single-person home...possibly with a senior, or a retired widow/widower. Something like that. I would eliminate as many variables as possible for Ophelia. No kids, no cats (although you mentioned she loves male cats...but in reality, it may just be Dory), a quiet, single-person environment. No dogs. Ophelia may do very well being the queen of the castle, if you know what I mean, and even better with 1:1 human interaction.
I'm thinking of you. I know you're struggling, and we all feel for your difficult situation. Take care, and know that what you're doing, is being done out of love.