Well, I got a set of easy-out from sears and drilled out the broken screw bit. Even though I didn't have enough clearance to set the drill straight onto the screw, it started backing out as soon as the easyout had some bite into the broken screw. I took a closer look at the broken piece after it was out and guess what, the screw is made of aluminum. This explains why it couldn't even handle 20Nm of torque. I purchased a set of replacement from MB and the replacements are steel. It kind of puzzles me why MB chose aluminum screws.