First, I'm sorry to hear about the loss of Branwen. I'm sure that it's still very hard for you and Nala.

Have you thought about getting Nala a new canine companion? Maybe she's lonely, since she was used to having Branwen around? I agree that leaving her in the house is the best idea--you could get those pee pads for her, especially since she's an older gal. It's great that she's in decent health.