So far (knock on wood), I think we've managed to find a solution to the food situation for our cats. We've gone with an all wet diet in order to have the low carb + high protein that Sam needs for his diabetes as well as the high moisture that Merry needs for her urinary issues. (Poor Spooky is just along for the ride; he's good as long as we don't feed him beef.)
Sam goes back to the vet in a couple of weeks to do another glucose test to see if we're actually being successful. I will say that I've noticed increased energy from Merry lately, so surely it's doing SOME good, even if it's just for her.