I am so glad she's home.
I have a friend who took in a senior kitty when he moved to a new home- this cat was left back by her former owners and was never used to a litterbox. I think the PT recommendations were to keep her in a room with a litterbox for a certain time- plus adding some soil to kitty litter.
In the case of this kitty all worked out fine (she was very old, so she passed away in the meantime.)

I think if you start a new thread you'll get many good advice.