Only one more, maybe. If you can see the carbon surrounding the center section of the plug, you might try digging into it with an awl or a small screwdriver. If you can get some of it removed this way, the penetrating oil might then be able to reach the deep part or, depending on how deep it is, you might actually reach the bottom of the plug.
Be careful in digging.....try to stay close to the glow plug part and avoid contact with the surfaces of the engine as much as possible. Even if you are successful in removing the plug, you are going to have to remove all that carbon before installing new plugs. There is a special reamer tool to remove carbon from glow plug holes. Expensive, but very well may be necessary.
Again, hope this might be of help and good luck.