The "drill-out" or EZ-outs require some finesse to get to work properly, particularly on frozen/stuck bolts. Have you tried something like an electric hammer drill set on reverse with the proper size bit (or by now an oversize bit?)

If you go with a drill-out approach, I'd recommend re-tapping the holes with the appropriate sized metric tap to clean out the threads. Ensure the threads are correct with the replacement bolts on a nut or off-the-car piece of metal before going this route.