A vet usually wants a rabbit to be brought in for surgery as soon as the office opens, although they may not perform the surgery for a few hours.
I hope it was the receptionist who told you not to feed Marshmallow after midnight, and not the vet who said that. Receptionists are often thinking of the protocol for dogs and cats, which is different for rabbits. A vet should know better and not tell anyone to fast their rabbits before surgery. Marshmallow should have access to food and water right before you take him to the vet. You really don’t have to prepare anything the night before. Take some hay and veggies to the vet’s office for him to eat after the surgery, and make sure you have all of his favorite veggies and fruits on hand when he comes home, in case he doesn’t want to eat much. If he doesn’t eat at all after coming home, you will need to give him Critical Care or a pellet mash.