Just have to try a little longer.

We have decided that we just cannot take Missy back at this point. She isn't a bad dog in any way, and we are figuring out how to keep them separated so that she and Jessie can't fight. It'll be a lot more hassle for us to keep them completely apart, but it will give us more time to work on things, and so on. So, I'm about to embark on attaching the X-pen panels to the walls so that there will pretty much always be two barriers between them. I can live with hassles to keep a dog we love.