I am sorry to hear about Tess.
My name is Rebecca. I am a veterinarian, and will do my best to help.
I would be worried about something wrong with her spinal cord. DM is a possibility; it is a progressive disease and it may not be advanced to the point of losing all feeling in her toes.
Dragging the hing legs, wearing the nails, fecal incontinence all point to something wrong with the nerve function in her hind end.
Other things can affect the spinal cord, such as a tumor in the cord. That would also cause a sudden decline. The only way to tell if there is a tumor, or a clot, or something in the spinal cord is an MRI or some imaging of the spinal cord.
The website seems to be not working right this morning. I hope you are seeing what I am writing. I can try to connect with you another way.
could DM really be as fast as this with a visible decline every day,