Actually diazepam should have a positive affect on a cats appetite. Vets often use it as an appetite stimulant in cats. If your cat is not eating that is a problem. Do whatever you cat to get him to eat- warm up his food, offer him treats, tuna etc... If a cat goes more than 3 days without eating he can begin to suffer from Hepatic Lipidosis which will make him feel worse and can kill him.
Atropine is used in these cases because it increases heart rate. Pet store type spot on treatments cause the parasympathetic nervouse system to become "overactive". The Parasympathetic system is our "rest and restore" system (vs. the sypathetic system which is our fight or flight system). The side effects of this type of overdose are muscle tremors, drooling, diarrhea, slow heart rate, slow respiratory rate... The heart can arrest in these cases which is why atropine is used to increase rate.