Just to establish my credentials, I have worked in some form of employer-sponsored benefits/HR since 1979, and have done benefits exclusively since 2008 or thereabouts. I have been the appropriate representative for the employer, the insurance carrier, and the third party administrator. (I told one TPA, when I was working for the employer, "I'm your worst nightmare")

I would say about 98% of the time, the insurance carrier bills the employer on a  monthly basis, using the calendar month. Let's take a specific example. The bill for June will be sent out somewhere between the 15th and the 20th of May. The bill is due on June 1, but the insurance carrier will accept the payment at any time during the month. However, if payment has not been received by the time billing for July goes out somewhere between the 15th and 20th of June, the carrier will remind the employer that June is still outstanding. In my experience, which is considerable from both sides of the desk (yes, we fired the billing analyst who let the insurance lapse), the insurance carrier will wait for June's payment until the end of July. If they've still not received June's payment, they will cancel coverage retroactively back to the date they were last paid for - May 31.

So now there is no coverage for June or July. We could go on for some paragraphs as to what happens if the employer now pays up, but since that's not what happened we'll leave that for another time. The insurance carrier has now paid out claims for which they have received no premium. No, it's not the fault of the patient/employee, who did nothing wrong and could not have known that this was going on behind the scenes. But it's not the fault of the insurance carrier either; they're the ones who got stiffed. And they have a responsibility to their entire clientele, not just to the one employee or even the one employer. So while some carriers may choose to eat the claims they've paid out, others will go back to the providers (the dentists, in this case) for refunds AND THEY HAVE EVERY LEGAL RIGHT TO DO THIS. This is not any kind of legal subversion, as you put it - it is a guarantee that their contract with the providers grants them this right,

So now it is the provider (dentist) who hasn't been paid for his work. So of course they're going to come after you. And the bottom line is; YOU received the service - it is ultimately YOUR responsibility to pay.

Your beef is not with the dentist; nor is it with the insurance carrier. They did nothing they were not entitled to do. Your beef is with the employer. You want to take legal action against them, fine; take it out of their hide. That's as it should be. The employer is the one who screwed up. But it's not the responsibility of the insurance carrier to eat the claims OR to contact you OR to explain what happened. That responsibility lies strictly on the employer.