I third the suggestion. If this is the first rejection, there should be an appeals process. I would keep it on a courteous basis for now (e.g., You understand this is a less-common treatment and realize they may need the opportunity to look over the data in more depth; you want to give them the opportunity to do so, etc.). If you did get to the point where you had exhausted all the appeals, then you could start looking beyond the company (e.g. state atty. general's office or insurance department or whoever regulates insurers in your state; your senator or representative, etc.). But I think you really do have a chance of prevailing if you just work through the standard appeals process within the company. If you need extra data, this recent review might be something useful to attach to the insurance submission:
On another note, she gained 17 pounds! Very impressive! I bet she's starting to look like a teenager, and is very happy about it.