This play can be considered to be about the "reversal of the natural order". What hints does the first act give in that direction?

Certainly the opening witch scenes shows abnormal order. The weather is bad and the witches uses dark spells and animal spirits to practice their dark arts. Macbeth becoming Thane of Cawdor is another disturbance in the natural order of the universe. The thane of Cawdor betraying his king was considered to be an act most unnatural and abhorred to God. Lady Macbeth wanting to be a man, so she can do bloody deeds, would have been considered most unnatural as well.