The key question is proving what lawyers call a causal relationship between the actions of the McDonalds restaurant and some injury. There is no question but that you have an injury, and based on what you say, it's getting worse. But you have to show that it was caused by the french fries and not some prior problem with your teeth. That's where the dentist comes in. I suggest that you talk to a lawyer. Considering that you have clear damages, the only question being causation, a lawyer...
You need to consult with an attorney in your state. While I am not familiar with Utah law, I highly doubt that the insurance company's reason for not paying for your wife's claim is valid. Most likely, they are just trying to get rid of you and hoping that you abandon the claim. Here in MD, where I practice, a claim such as you describe would be worth some amount of pain and suffering, depending on the particular facts of the case. Some cases involve a great deal of initial pain which then...