That pre-existing thing is basically criminal. Insurance companies use that and recission to basically avoid paying what they're obligated to pay.
Sadly, I've gotten good at this, as my wife had a six month fight with cancer. But with time, I successfully got the insurance company to pay most of what they owed (the majority of the rest, I got the hospital billing department to dismiss). It's about persistance and never taking no for an answer.
Bottom line, contest. And the insurance company will come back and say "We asked around the office if we owed you money, and everyone said no, so we processed your claim correctly". Then send them another letter (ALWAYS get everything in writing) in which you use the words "medically necessary" and possibly "negligent". Note that Alison's career and future income depend on the correct processing of her claim. Reiterate that it is NOT a pre-existing condition. Letters from doctors help.
Finally, if this fails (and it might), get a lawyer to draft another letter. Know any videogame fans with a law degree? Might be able to swing probono (I did). Send a letter describing the insurance company's negligence and intransigence to the state insurance commissioner and cc the insurance company. Lessee...top notch graphic designer's career up to retirement age is probably worth several million dollars. Yeah, that might get their attention.
Bottom line, never let the insurance company dictate the narrative. The only successful and proper outcome is when they pay what they owe. Any other answer is a non-starter.
Gosh, can't tell I feel strongly about this issue, can you?