Sorry to hear about ur car and what u have beeen goin through man/
First,ive had a SC on my car for the past 20K miles and ive yet to have a single problem.
Second,the Subframe in the E9X M3 is no joke,aint like the E46 M3 aqnd E36 M3 and so is the differential.
I think that u might have had something wrong with ur subframe when ur car was built and the differential just went bad cause it wasnt mounted right to the subframe,may be it was off so it wasnt spining freely.
Dont take its not covered under warranty for an answer and good luck man !!!1