OOC: I agree with you on those points, the question I have is when does the AoO occur?
1) After Sandy has reached -5Z, in which case she has full cover.
2) While Sandy is in the process of moving from 0Z to -5Z, in which case she has not yet achieved any degree of cover.
If Sandy was going around the corner of a hallway, and she was leaving a threatened square that did not yet offer cover, I would rule that the attack would occur at the threatened square, not the square that offered cover.
Is there anything I'm missing here?