Don't know what to tell you.... I had this happen to me before (about 20 years ago) on a previous vehicle. And after the shop that had it (quite reputable, I should add) fiddled with it for about a week trying to work the broken plug out, they ended up just pulling the head off and getting it out that way. Then they installed a helicoil.
Sorry I don't have better news, or words of wisdom.