I understand you are probably very frustrated right now and may feel trapped, I would too, however you need to look at this from the rep's position as well. I work from AT&T will tell you that if you were only listed as an authorized user on the account, and didn't know the passcode, I wouldn't give you access either. If it hadn't been you that had called, someone else, and were alllowed complete access to YOUR account, you probably would not be very happy. Those policies are put in place for your security, to limit unauthorized access to the account. I can't speak for the other reps you say hung up on you, however I can tell you why you were not given a callback-you couldn'y verify the account, if the caller does not verify, than for security purposes we can't call you back. Remember when you call customer service you are on the phone, the reps can't see you, have no idea what you look like. At this point your best bet would be to go to an AT&T corporate store, not a retailer, an actual corporate AT&T store, take your driver's license or other form of government ID with you, along with some sort of official form, tax documentation, bills, something to prove that you do own that company, and if you do have an old AT&T paper bill or something that includes your name and account number, take that too.
As far as this other employee whom you've stated put his own name on the account, I don't know how that could have happened, and if it has been proven that he did so, is probably no longer with the company. However, calling him at home, going to his house, making demands in person, that will do you no good, but may get you in trouble for harrassment. Calling him on his "off hours", making threats that you have his address, that can all be seen as harrassment or even stalking and the only thing you will get then is a possible prison sentence.
I know you're frustrated and want this fixed, but take my advice, go to the corporate store and let them handle, they can physically see you and any documentation you bring, whereas the call center reps cannot. Leave the other employee whom you say did this alone, if it is proven then there are other legal ways of handling it, and may have already been when his supervisors found out what he had done.
I hope this all works out for you, just be patient and remember, don't blame the other reps you spoke to after the fact, it was not their fault that this had happened.