To everyone in a similar scenario: the tactic my doctor's office has taught me is to ask, in writing, for:

1. the name, board specialty, and license number of the doctor making the determination the treatment was not medically necessary;
2. copies of all materials they relied on to make their determination;
3. proof the doctor making the determination has maintained registration in your specific state and documentation of their meeting all their continuing education requirements;
4. the aggregate rate at which similar treatments are denied vs approved by the specific doctor being used for peer review.

You are not entitled by law to all of these things in most states, but you're entitled to some of them, and you can always ask for them.

This is, she says, a wildly successful tactic, because if the insurance company answers them honestly, it gives you evidence that the "doctors" making these determinations are practicing medicine out of scope, without proper licensing and qualifications, in areas they are not competent in.

Everyone knows this is true; it's not a secret in any way. But it's in violation of a number of regulations, and a LOT of times the company will just give up and pay the bill rather than handing you proof they're violating the regs. It's a tactic that has worked for me many, many times.