OP. As I said from the beginning, make sure you're running identical BRAND and MODEL of tires front and back as well as the CORRECT BMW STOCK SIZE for your car. Don't mix and match brands and models and wear patterns. If you buy two new tires for one axle, make sure you get new ones for the OTHER axle very very SOON.
Not quite sure exactly how someone knocks off a speed sensor when doing tires. Is that what you are saying happened?