What do I need to do for AT&T to fix my plan without forcing me to resort to ridiculous tactics?

I pay for my mother and sister's phone and have had an AT&T family plan for many years now. Back in March this year my sister broke her phone and needed to get a new one. I added her as an authorized user so she could go to her local AT&T store while I was busy at work and get a new one. I told her she had to pay for the phone herself but it was okay if she signed a new contract to get the discounted rate. 

Somehow in the process of hooking my sister up with a new phone & plan the AT&T retailer managed to deactivate my mother's phone. A couple days later my Mom called me to ask if I cancelled her phone. I immediately called AT&T (611) to figure out what happened. After hours on the phone we ended up having to conference call the local retailer to try and figure out what happened. They clained their system was "frozen" and no managers were available to help. In the end AT&T conceded that the retailer messed up and they re-issued my Mom a new sim card.

I had a feeling my bill would be interesting the next month and sure enough it was. AT&T was trying to charge an activation fee for my mother's line etc. Once again I had to call and spend more of my time to get it resolved. They ultimately agreed it was an error & removed the fees. 

Fast forward to the other day. My mother's birthday is approaching and I am considering getting her a new iPhone. However, I notice that online it is saying my mother's plan doesn't expire until 2016!? The same exact date as when my sister's new plan expires. My mother has had the same phone since 2012 and should qualify for an upgrade by now. Obviously this is related to the boondoggle earlier this year. So I call and once again spend hours on the phone explaining how this was supposed to be resolved back in March. The rep was actually very nice but was not able to offer me an adequate solution:

First he said I needed to go, physically, to the retailer and buy my mom a new phone and they will correct the contract mistake in the process. I explained how ridiculous that option was. I can only fix the false contract by signing another contract? No. So he said he would call them to figure out what happened and call me back. When he called me back, he said they told him they were "busy" and could not help him. So again, he said I needed to physically go to the retailer to figure out what happened. Again, not acceptable. The retailer is totally incompetent and I will not allow them to further mess up my account. I have already invested several hours of my life getting this issue resolved back in March and there's like a million notes on my account confirming the whole situation. There is no need to contact this joke of a retailer again.

He then agreed to keep trying to call them and would call me back on Monday. I have no clue why he keeps insisting on calling them again considering the history and notes on the account. Anyhow, he calls me back today (Monday) and informed me that the retailer said they couldn't find anything in their system (or whatever their excuse was) so the only option left was for me to call the retention line and basically threaten to cancel my account and hopefully they can correct it. Are, you, freaking, kidding me!? I have been extremely patient so far but I'm about fed up. I asked to speak to his manager and of course they were going into a meeting and would call me back right after. This was around noon today and still no callback...

What do I need to do to get AT&T to take responsibility for their mistake and correct this once and for all, without forcing me to resort to stupid tactics? If I have to call retention it will be to cancel my account for good, not to fix my plan which was already "fixed", twice.