Sharpen a pristine point on a small center punch or prick punch and drive the punch into the face of the allen head about half way in between the outside of the hex and the edge of the fastener.
Drive the punch at a 45 degree angle on the right side of the face (to reverse) to rotate the fastener out.
That’s a great idea for the next to last resort. I’m going to try lubricants and heat before taking it to the next level.
Thanks.