Toni, the first thing that comes to mind is that Dark needs a companion ktty to heal his heart.

How about trying a new fuzzy toy (rubbed with catnip)?

Does Dark "know" that Tall has passed? Our vet suggested we take a small hair clipping from our Gandalf when he crossed the Rainbow bridge, she said to let our other cat smell the hair and that would let her know he had passed. it did seem to help.

It certainly sounds as if he is missing Tall from his using all of Talls beds, etc.

I tagged Beth Bach at the end of your story in the hopes that she might have some ideas. Beth is a very experienced cat rescue person.