That bolt stub should remove fairly easily with a little luck. The torque requirement up to the point of breakage is now gone. So the boken bolt still in the block should be pretty loose. There is absolutly no loading on the threads now.

Yes it has to come out and be replaced. I really feel you will be able to do that with the head on unless something is really wrong. Do run some form of bottoming tap down when you extract the remainder of the bolt. Blow out the hole as well. The bolt did thread in as easily as the others until the cap portion touched the head?