Time for an update.
Mai's EX air qcb+AC does NOT have invincibility properties, thus it should be used as an offensive maneuver. Three hits and 0 frames on block. Sense Mai is neutral on block from using EX air qcb+AC, she will have plenty of options available. Should EX air qcb+AC hit successfully however, get ready to combo afterward.
From mid screen, you can input in a hcf+B and then drive/super cancel after the last hit.
Use EX air qcb+AC for mid air mix ups, set ups, keeping things safe (again, this move is 0 frames on block), and starting combos.