I do feel badly for you and your Chihuahua. Obviously, you know what you're doing as your post was concise, informative and articulate. How refreshing!
Lucy has a point, and it would have been my suggestion as well. Since you, "BruisersMom" had already mentioned the Vet, I do think it's a good idea. When nothing seems to have changed, except their behaviour and you've tried everything that you know has worked in the past, it is time to see the Vet, especially with a dog that is 10.
Please keep us posted as I know this is not very common, and perhaps your experience, like Lucy's, can be helpful to others. Besides, we do want to know how you are managing. You must be absolutely exhausted.