Sorry, I missed your latest updated post. So my previous posts was a little off the mark. From what you are saying and have been reporting, I am going to venture a suggestion. Now this is just a suggestion and please keep in mind that I could be totally wrong.

Nook doesn't seem to be suffering (at least from what you describe). He seems happy and alert and has good days too!! He is eating and has control of his bodily functions (I mean he's not having accidents everywhere). He's attentive and responds to you intelligently. I say he's just old! Let him be OLD for a while longer and don't fret over his spills and mishaps. Remember when humans get old, similar things happen. When he is ready, he will probably go downhill fast and you will know. He'll stop responding to you. He'll be indifferent to food, attention and hugs. His spills may be morre serious and if he is unable to recover quickly and perhaps crys or wimpers at night - THEN YOU'LL KNOW. Good luck!

Dan (Bob's Dad)