Just going back to physics, forgetting that it's a BMW, isn't the cause of chatter the amount of pressure being put on the blade? i.e. if you gently push down on the blade with your finger, the chatter goes away?
BMW service truly is stupid at times. If you guys have another brand of car besides BMW, you'd be surprised how you're treated in the svc. dept. It's very refreshing to tell you the truth.