Maybe you can return the other unopened cans. They are all too high in carbs for a diabetic cat. You could always try something like all meat baby food. Beechnut and Gerbers make them. Cats usually like them and they are OK for a few meals, though not complete so not good for long term.
Too bad they gave you Mirtazapine. Mirataz is the same drug in ointment form that you rub in their ears. It's much easier to give and less likely to have side effects. It's an appetite stimulant. Make sure you give it at a separate time from the Cerenia. You don't want to give an appetite stimulant to a nauseous cat.