Dental Claim denied -Provider Error

Question.. I went to the dentist about six months ago and I needed a root canal from about a decade ago to be retreated.. a receptionist at my dentist entered in the wrong insurance eligibility dates into their system for my insurance. So they treated me 22 days before the end of my one year wait period for any procedures over $250. My insurance denied the claim, I already paid my portion – $598, and insurance was supposed to cover the other 500+. When I received a denial and I dug into the facts, it was discovered(and confirmed by my dentist office) that they entered the dates and wrong and that’s why they scheduled it. I even asked to make sure it was covered before I schedule the appointment. And they said yes. They said I would just have to pay my portion, the 500+ I paid. My dentist office said to submit an appeal and that if they didn’t accept the appeal, then they would work with me on a payment plan. ?! I appealed it with my insurance company, and they denied it. I was gonna submit an appeal again, but was already told by my dentist that I would be liable for insurances’ portion if they deny again. The mistake was on the dentist office and they’ve already admitted fault, but also said that if my insurance company doesn’t it then it’s gonna still be my responsibility to pay the difference because when it comes down to it, it’s my responsibility to know about the one year wait period. My father is a lawyer in Ohio but he can’t really give me much guidance in North Carolina. Can anybody help with this? 600 hundred $ doesn’t seem like much but it is to our family in this economy.