wraith no, since they have shambling.
another unit with fly however, can move at the double if they do not have shambling.
fly over and turn 180 degrees - if they move at the double, no pivots are allowed. if they have the nimble rule however, one pivot of 90 is allowed.
if the fly over with normal move (not at the double) they are allowed one pivot + one if they have Nimble. these two can indeed be combined into a 180 degree turn by flyers who are flying over things. if the flying unit took any damage in the previois melee phase however, they cannot Fly, so any pivots must be turned and end in a legal position (90+90) and can not be combined into a single 180.