Choppy tire wear is usually the result of worn suspension parts creating an uneven force on the tires when turning. This could be from worn or broken bushings, tie rods, or worn shocks and struts typically. The other big cause of uneven tire wear is a misalignment, choppy wear typically from a bad toe in or out.
Uneven tire wear shouldn't cause any kind of grinding noises, but may cause vibrations and usually a humming or roaring noise. I would probably stop going to BMW dealerships and go to a reputable independent that will check your suspension and can do an alignment with the proper weights and specs for your car.