So sorry that Poppy not well - I'm sure we all want your poopy to get well as soon as possible
They are really fragile at this age, so if you are at all worried, I would go back to the vet - they can dehydrate so quickly
Try to tempt her with some cooked chicken, mince or similar - at this stage, even a small bit of food might just get a bit absorbed, so will do her good
Puppies do sleep alot anyway - and the combination of drugs she has had might be a bit overwhelming for her and so make her sleepy too. I am sure that once she is feeling better you will be looking back at these few "quiet" days
Only you know if taking her back to the breeder is the right thing to do. Some pups do get very stressed when they move into their new home, and can get poorly to a lesser or greater extent; but ear mites don't sound right for a puppy from a reputable breeder
Did she come with Insurance, from the KC, maybe? If so, you will probably be able to claim on that for the costs of her treatment; but bear in mind when you come to insure her after the "free" period, that this will count as a pre-existing condition so you will have to declare it if you move insurers :-/
I hope you and Poppy are feeling better soon