A Station Eight Fan Web Site
So I know you've answered this a number of times over the years, but rather than I asking the nth time, I've spent a while thinking about the "how".
So Macbeth and Demona cannot die but by their own hand and although there are situations that seem like they could die by another's (beheading, smashed stone, etc), these situations cannot happen because of the spell the Wierd Sisters placed on them. It protects them from assured fatal injuries that normally would kill mortals (again beheading, smashed stone, etc). The spell basically would manipulate events to ensure that Macbeth and Demona would always get out of such a situation (Macbeth getting caught in the French Revolution and is scheduled to be beheaded but some mishap with the dungeon keys delays it, buying him time to escape or Demona is forced to roost elsewhere instead of her normal spot because of some freak storm preventing her in getting back, thus sparing her from being smashed by the Hunters one morning).
They would have uncanny luck in avoiding death situations that would otherwise be assured.
I guess you're basically right, but I would recharacterize it. The Sisters may not have magically enforced this "manipulation," as you put it, so much as they magically predicted future events.