Problem 2: The five cats are on four different diets. All cats, of course, prefer the diet they are not on, and they will swap as soon as they think I'm not looking.
Problem 3: Two of the cats, Butler and Daisy, very much need to lose weight. (Chess could stand to lose some as well, but she's a picky eater, and she at least goes crazy. Butler is the laziest cat in the house.)
Tentative solution: Playing with food.
Butler will dive into his food, eat half, and leave. He's a gentleman, you see, and he knows that Chess is just picking at her prescription food until he leaves, so that she can swipe some of his prescription food. Then, once she's finished his, he'll plague me for more.
They tried that routine tonight, and I caught them fairly early on. Chess was quite put out at being made to go back to her own food. Butler acted as if he had been deprived of half his meal. He can be a pest, so I decided to give him ten more pieces -- but he had to work for it. Rather than putting the food in his dish and leaving, risking Chess taking that part too, I kept hold of the food and called Butler over. Then I rolled one piece away across the kitchen floor. Butler caught on swiftly, and by the fifth piece he was having a high old time. He was having so much fun, in fact, that Chess came over to join in the fun ... but when I tried rolling a piece of her food, she got insulted.
It's not a perfect solution; it takes time and attention, it encourages him to toy with his food, and it risks food getting trapped in various corners. But it gets him to exercise and it keeps his food going to him rather than Chess, so I'm willing to give it a try.
I just need a way to try this out with Daisy. I don't think rolling a food pellet across a carpet is really going to work very well.