Just another $.03 worth
Several of the above threads provided the right info to get the job done or at least put you on the path to a repair or replacement. If you haven't ever removed the elbow before, then I doubt if it would leak if you pull all the bolts out. These little thermostats they have for these MB have a rubber o-ring seal that surrounds the thermostat and after a week or so of driving they pretty well seal to the aluminum housing. I know, it took several blows from a rubber mallet to even free mine after the bolts had been removed. You simply have a few choices as was already mentioned, but don't worry about it leaking in the mean time, it won't. I like Bill's idea of finding a source for the elbow, in case you do need it, and then remove the other two bolts and let some penetrating oil sit around the threads of the broken bolt for an hour or so, then heat it up and simply remove it with a vice grips. Make the grip tight the first time though, so it doesn't chew up the remaining stud. The easy out method will also work if you don't use a chineese made easy out, but one that was actually forged out of steel and then hardened. Like bill says, the elbow can always be replaced if all else fails, so you're not SOL on this repair. I'll trade you repairs, I'll do your elbow if you pull my IP and retime it?