First off , I would go back to the dealer that charged you that OBSCENE amount of money for a diagnosis and "repair" to have them give you a detailed account of what they did and why. You can then take that to the other dealer and show them what was done and ask what they would have done (see if they would put something in writing in the form of an estimate). Then call the GM Regional Office and let them know what went on and ask for them to resolve the problem. From your post, it sounds like you still have the problem.
Work only with the Service Manager at the dealerships Be firm in stating you believe you were overcharged and if the tranny still does not work right, ask them why it does not. If they blow you off, go directly to GM to start the resolution. Get names, times you talked to people and make sure you have copies of your repair order and invoice.
I don't know a lot about A4's but there are some solenoids that are plainly visible once you remove the pan. You might also get estimates from local reputable tranny shops to see what they would charge and whether or not the tranny would have to be removed from the car.