My tough one was #4, the one that had failed. In a nonop condition, the tips carbon up pretty quickly.

I used a chunk of angle that I cut a strip of that was bent out at a right angle. I left the main part of the angle long enough to reach the head and create a fulcrum. In the bent out leg, drilled a hole the same size as the plug body and then cut the leg off through the hole creating a c-shaped "cradle.". This fit just under the hex head and allowed me to turn the plug while applying gentle outward force on the rotating plug. The was just enough to "saw" off the carbon and pop it out. Making the "tool" and getting the plug out took 20 minutes.

Unfortunately, I spent 5 hours messing with it before getting to that solution.

I had one break in the head. I bought an extractor kit from ebay and successfully removed the stuck part of it without removing the head

Needless to say be very careful and take your time with any forceful extraction. May also want to try something like Mercruiser Powertune or Yamaha Ring Free (yeah boats are another of my vices). Both are pretty good at breaking up carbon deposits. Just remember that anything you put in there to help release the plug has to come out before you try starting it up.