I'm so sorry that you've run into all of these problems with this breeder. How strange that she would give a cat's vaccination to a yorkie puppy.
Based on what I can see in the pictures though your little yorkie boy looks like a purebred to me. He's very cute. I do know several friends that have yorkies weighing up to 20 lbs so it does happen. I agree they can come in all shapes and sizes. It's possible that the parents of your puppy were still carrying genes for larger yorkies from previous lines which they carried on to your puppy.
Everyone has given you great advice here and I hope this all works out for the better for you and your puppy.
