If you get a second litter box, is it possible they would each use one so you could monitor the poops?
When I was growing up we had two sibling cats. They lived 18 years on Alley Cat dry "food". One was hit by a car at that time, and the other disappeared just a few days later...who knows how long they would have lived otherwise...? Using the Z/D diet is the cheapest and simplest way to figure out if your cat has an inability to digest proteins properly. I'm sure not one to say the food is good, but I highly doubt it would be harmful when used temporarily. Following the vet's advice seems like a good place to start imo. Oh well, we will see what you accomplish in the next few weeks. Hopefully your plan works for you.