Man that sucks! I feel for you. I've found that there's so much Loc-Tite (or whatever the MB factory equivalent is called) on subframe bolts, it's a miracle that they ever come out in one piece. I can't offer any advice other than to do whatever it takes to get the broken bolt out. We don't do many subframe bushings, but when we do, it's almost always necessary to repair the threads in the body, which tend to get torn up pretty good when the bolts are removed.