Now that I'm in Medicare I don't have to worry as much, and now my problems are accumulating (75+).
So many stories about patients contesting out-of-network charges are rampant. Also many discussions about how to dispute charges are effective - most patients don't try or give up after a first rejection.
First, I think you should make sure you get all communication in writing. Don't take the word of a phone-answerer as what might be honored. Get documentation and authenticated communications.
Personally, I would also record any phone conversations with any insurance companies, etc. I know it might not be admissable in some states but you'll have the recordings for use to back up your own position.
Secondly, find a health care advocate. I haven't had to get one so I don't know where to suggest. Perhaps your state's human services agency may have a list. Look for ombudsmen, perhaps hospital care navigators.
Claims processing is notoriously sloppy - even for the insurance companies and the recipients. Individual charges can take months working their way through the paper handling.
Maybe consider writing a few friendly but accurate and forceful letters to the upper-echelons of the organizations involved. And think about writing some personal comments to local papers.
Organizations like KHN and Propublica as well as others are very receptive to getting individual stories. If it is part of a trend then they are very powerful at getting positive action.