the 'king's chamber' shafts were completely open to the outside (though whether the casing stones sealed them off we may never know)
I'd have to say you're correct. As I understand it, the shafts leading from the King's Chamber were not plugged. But it's a good point about the casing stones--we can't know if they covered the outlets of these shafts.
Just the same, whether or not the shafts leading from the King's Chamber reached the outside unimpeded, I would still caution against seeing them as "air" shafts. This is a modern appellation--nothing from ancient times demonstrates such a thing one way or another. All we have to go by is the context of the Great Pyramid in comparison to other tombs. And of all the royal tombs constructed over the span of 3,000 years in ancient Egypt, I can't think of any other with something similar to the shafts inside the Great Pyramid. That goes for private tombs, too, come to think of it.
Logic dictates that if these shafts were made for the ventilation of chambers, many if not most other tombs would have them, too. And yet they are absent in other tombs, so logic dictates they were not for the ventilation of chambers.
Whatever the case, claiming they were for ventilation still makes more sense than trying to claim the ancient Egyptians used light bulbs.