I would rule out anything medical, then try kitty prozac. Ditch the couch because now it smells to Luna like its a good place to go.

Maybe after she has started using the litterboxes again to consider another cat but only if you and your vet are sure she won't start her behaviour problem again.

As for litter boxes, I now usea different litter for Bastet because the one she is used to Ican no longer get. I have always used a variety of litters though because of the fear of cat wee in the house I am living in now. *My flatmate's house OWNED by him and he would KILL me first then the cat *

I had to go from an open box to a closed box with Bastet. I think she likes privacy.

Best of luck.