What's the current plan of attack for the broken bolt?
One option is to look for a shop in your area that has what's known as a "tap burner"- it's basically an old fashioned, coarse EDM machine, mainly used for exactly what the name implies, spark-eroding out broken taps, bolts and EZ-outs.
The vast majority of them however, need the head loose- it has to be mounted in the machine. A few, however, have the capability to be mounted to the part.
It's certainly worth asking around.
And, if it turns out you have to pull the head, then by golly pull the head. You can take the opportunity to give it a good cleaning, scrape out some of the carbon, give the valves and seats a quick lap to restore a bit of sealing, and get a fresh head gasket in there so you hopefully won't have to worry about that blowing out.
And good lord, am I still your one and only Patron? Hey you guys, throw the poor boy a bone, would you?