I'm sorry to hear about all the trouble you're having. I didn't have this much trouble getting my insurance company to approve my BAHA, but I know others who have been through this. If it's any consolation, I can tell you that if you keep pursuing coverage, chances are good that you will "win". Most insurance companies think that people will give up after the first denial - after all, it saves them money - but if you persist you'll get covered. Don't take no for an answer.
Believe it or not, you may have to educate your insurance company about what a BAHA is and why you need one. Most insurance company personnel and doctors have no idea what a BAHA is; even my ENT was clueless when I informed him I had gotten the implant and amazed when I told him how it worked. You should ask for a peer to peer review of your request. This will insure that a doctor who does BAHA implants will actually review your request, rather than a doctor who doesn't. A peer to peer review is something you are entitled to.
Also, although it sounds like you already know this, it's important to make sure the BAHA is coded correctly - it's not a hearing aid, it's classified as a prosthetic or an implant. It also doesn't hurt to stress that a BAHA will give you a better quality of life and also alleviate (potential) depression on your part due to lack of hearing.
Getting a BAHA implant after my AN surgery left me SSD was the best thing I've ever done. I was miserable and so were my kids who hated to hear me keep asking "what??"
Hang in there and don't give up; it's worth it.
Best,
Jan
Forgot to mention that you should see if your insurance covers Cochlear Implants (CI's). CIs are for those who are bilaterally deaf. If your insurance company covers CIs and doesn't cover BAHAs you can claim that they are discriminating against you (because they are) since they are covering those who are bilaterally, but not unilaterally deaf.