Oh wow. I am so sorry for what you've been going through. I know how hard it must have been for you to do what you had to do. But think of it this way, Sherpa and Charlie are happy cats now. They are not stressed out and living in fear of the "mean kitty lurking around the corner".
My two do not get along after 3 years, but there isn't inappropriate urination or any stress from either of them. And I've seen Paizly stressed when I was fostering a friends cat.

I think if Kyubey and Celeste get along then by all means keep her. You might feel like you failed Charlie and Sherpa (which you didn't!!) but you could give Celeste a great home. And I know your home is a great home! After all, look how Paizly turned out!