Regarding broken extractors. Use the hardness in your favor. I've had success using a pin punch and small sledge to break them into pieces which you can then pick out. Use safety glasses and resharpen the pin punch on a grinder every few blows.
If the threads are messed up you can always tap it to 1/2 x 13. 1/2inch is close to 13mm which is slightly bigger than the 12mm hole you have.