It has been a while and no one responded to your problem, so I thought I would take a shot.
If the plugs are sheared off, and the head is off the block and out of the car, I think I would try drilling them out at a good machine shop.
Buy a new plug and take some detailed measurements to find the minor diameter of the threads. That would be the diameter in the root of the thread. Then measure the other diameters, and sketch out a plan to drill and ream to the minor diameter, without cutting any head material out, including any of the bore the glow plug fits into. Once you get close to the minor diameter of the thread, you can usually peel the remnants out with a pointed tool.
Once you get the threaded part out, the rest should come out pretty easily. And you should be able to tell if the carbon deposits caused the problem to start with, or not. It sounds a little fishy to me though as the problem of not running and carbon deposits do not appear to have a similar time constant. One was abrupt the other is a slow build up. And the build up attacked a bunch of the glow plugs in the same time frame.
Good Luck, and rather than just give up and call the head junk, I would risk drilling the stuff out. The worst you can do is make it really junk.
1986 Euro 190E 2.3-16 (291,000 miles),
1998 E300D TurboDiesel, 231,000 miles -purchased with 45,000,
1988 300E 5-speed 252,000 miles,
1983 240D 4-speed, purchased w/136,000, now with 222,000 miles.
2009 ML320CDI Bluetec, 89,000 miles
1971 220D (250,000 miles plus, sold to father-in-law),
1975 240D (245,000 miles - died of body rot),
1991 350SD (176,560 miles, weakest Benz I have owned),
1999 C230 Sport (45,400 miles),
1982 240D (321,000 miles, put to sleep)