Need help with where to go after fighting with insurance for 6 months

Approximately 6 months ago, my health insurance (United HealthCare) stopped participating with the local healthcare monopoly here. My 5 year old is currently in remission from leukemia and has in office exams and blood work every 3~ months to ensure her cancer hasn't returned. Sometimes she needs these visits more often if a problem arises. Come to find out, this pediatric oncology office is the only one locally that sees patients as young as she is. The only other pediatric oncology group won't see patients younger than 12. The next closest in-network provider that can see her is 2.5 hours each way via car and out of state, or 3.5 hours on the entire other end of the state.

From end of December until now I have spent countless hours on the phone with UHC reps to figure out solutions, and every step of the way UHC has made errors that continue to cost us time and delays in care. It started with needing to obtain a continuity of care (which I did right away) which they messed up continuously to the point that I was never able to get it approved before the option to have a Continuity of Care was off the table. I can go into detail regarding the errors but I am not sure it's relevant here.

After that, I am told to obtain a gap exception. Over the course of 2 months, myself and her PCP office dealt with many more phone calls and emails to UHC checking for updates since my daughter was now behind on her cancer follow ups. We ended up needing to submit it a second time because they claimed they never received the first one even though I have documentation they did. Rinse and repeat and even after having it expedited, they eventually claim a gap exception was never submitted AGAIN. Now, the difference with this second round was that I got an approval letter in the mail for 9 visits through 04/24-12/24 and finally took her to the oncologist only to get a bill for 100% of the costs in the mail. This was when I submitted an appeal thinking surely UHC just made another mistake, but after getting a reply stating the appeal was upheld I called them back and they are claiming a gap exception was never received!! I have timestamped copies of all forms completed, sent, faxed, confirmation pages, etc. For both times. I know the gap was submitted both times with absolute certainty and even received an approval letter. UHC claims this approval letter was for a prior authorization, not a gap exception, and that the PA was approving us to see the out of network provider at out of network prices. This literally does not make any sense, why would I even waste my time for approval to continue what was already the case? I got a supervisor involved who stopped responding to my emails so now I have to reach out to them AGAIN and I don't even know what my options are at this point, even though I have documentation. I have never dealt with such incompetence and frustration in my life. This has become a part time job for me with the amount of paperwork and phone calls I've had to do for this.