We lost a puppy at 9 months old when he started having cluster seizures and did not respond to treatment. We went back and forth with the breeders for awhile, were offered a replacement dog and didn't accept it. My husband was good at reminding me to understand that it's a business for a breeder, a congenital defect like deafness (epilepsy in our case) could mean a sigifigant financial loss for them, they need to be 100% certain that it is what it is and I'd encourage you to get the documentation the breeder needs not necessarily to replace or refund your pup, but to allow them to make the right business decisions based on the defect. But at the end of the day we'd do it all again for the time we had with Coop and it wasn't about the money, replacement or honoring a contract. Sounds to me like you've been blessed with a special pup that's entered your life for a reason!