"So, when I backtest, I used historical implied volatility, historical interest rates, historical dividend rates and Black Scholes to calculate the option prices. How much can I trust the results since I was missing volatility skews/smiles and accepting the approximations of a Black Scholes. And are there better ways to backtest?"
I commend you on your effort, but damn you're making it way harder than it has to be my friend. Open an account with TD Ameritrade and use the ThinkorSwim platform. They have 5 years of data that can be trusted.
If you want to go back further than 5 years, you'll probably have to use a paid service, which are out there. You can get option pricing going back 10+ years if you're willing to pay a bit for it. For me since I've actually been option trading for nearly 20 years with 5 or 6 systems that haven't changed much over the years, I don't really need to go back further than 5 years on my testing but I've seen several people on this forum reference that they do. Perhaps someone will be along to give you the name of a really good service that dates back 10+ years.
For now, ThinkorSwim will do just fine.