You should have gotten a second opinion (or third) by a dealer a long time ago. Dealers are like doctors, they can all have a different diagnosis (believe it or not, not all techs are good at their job). After that, if you are still unsatisfied, call BMWNA and log an official complaint. Like you found out, they may not do much, but if you're lucky, they can send a regional tech to the dealer to also diagnose the problem and hopefully provide a resolution.
In cases like this, you have to get the dealer on your side so they can go to bat for you when they discuss the issue with BMWNA. Again this may take several visits to different dealers. Unfortunately b/c of BMWs generous warranty, they hear and see it all. I've seen driver neglect get rewarded and victims get left holding the bag. Dealers get reimbursed from BMWNA for all warranty work, and they don't want to eat the costs of something that may not be approved by BMWNA. Knowing your SA and tech (and hopefully a BMW Master tech) is also a good idea.