Were the threads chased?
As in did you run a tap down them to clean up the threads and make sure the bottoms of the holes are clean and free of anything that may cause the bolt to bottom out before intended.
How much oil was used to lube the bolts.
Look up "Hydrolock".
A small welder will do the job.
Get it out a practice welding a similar size bolt, washer and nut together.
A washer over/around the broken bolt shank will keep the block protected some and give the nut a little support. A large fender washer might give room to tac to outside the nut as well as the center. (make sure you can still get a wrench or socket on it)
Welding the threads to the block would take some effort with a small welder I think, but that's why I suggest the washer first.
I'd try using some wax as soon as the nut is welded on to take advantage of the heat cycle generated from welding. I haven't tried that technique but this would be a place to give it a shot if it were me.
