You are a normal Yorkie mommy worrying.
Make sure you have pre-op bloodwork done. Is your vet aware of the seizure a few months back?
Many vet offices do not have staff in the office overnight. The primary reason vets want to keep the dogs is that many owners won't keep the dog quiet after surgery. Many dogs bounce back very quickly, but they shouldn't be running around right after being neutered. If you have an xpen or kennel, you could insist that you want to bring your dog home before the office closes for the night.
Best wishes to Jax.