The remaining three non-sheared bolts were tight...very tight. So I applied patience, Kroil, and heat and vibration. Making sure the socket was buried in the bolt head...they came out slowly. So far so good. With the flywheel and washer removed, I had a decent amount of bolt protruding.
Several ways to attempt to skin this cat...
Reverse drill bits
Easy out
Notch the stud for an impact flathead
Std stud removal tool
Weld a short bolt to the broken stud
Vicegrips
I couldn't find my reverse bits (first choice) but used Kroil and heat and vibration again as prep. First one spun out with my fingers...second and third needed just a bit of tempting with a small vicegrips. What could have REALLY sucked was a non-problem. Pretty sure I was due....
Flywheel looks ok...for having bolt heads bounce around between it and the disc and the PP. Still not convinced that was the real story, but it doesn't matter now.
Thanks everybody for the guidance!