Dear Mr. Toxicant,
I'm sure you're not in the best of moods with a broken plug and the threat of pulling the head on your 500SEL, but the others are right in advising to use anti-seize next time. This is also why I wouldn't let any plug sit in an engine longer than 30 k miles, I don't care if it was made out of Kryptonite!
I had the tightest plug I've ever seen in my Volvo Turbo this past winter. The PO had overtorqued the plugs and I paid for it. I got three out of four out on the first try, but number three was tight enough where I couldn't get any movement after about one flat and mor than 100 ft-lbs. I started the long process of applying PB Blaster every couple of nights after I got home. After several soaks, it still wouldn't budge. I finally got lucky on my last ditch effort before taking it to a shop to pull the head. I soaked the plug hole again with PB Blaster then fired up the motor (which I realize isn't an option for you right now), and let it run until my hand didn't want to stay on the exhaust manifold. Then I was able to get the plug out by going two flats out, one flat in, more soak, two flats out, one flat in, more soak, etc...
The moral of the story was patience and running the plug in a little each time it moved a little bit more for me. This kept the threads wet, I think. You can't run the engine, but you can probably figure out a way to get a little heat on the head.
Good luck, let us know how you do.