*Original questions have been edited down to a single question (per eNotes policy).
From the very beginning in Act I, scene I, the witches' chant sets the stage for the theme of appearances versus reality:
"Fair is foul, and foul is fair" (I.i.11).
Much of Macbeth hinges upon false appearances and deception. The witches' contradictory chant suggests that objects or people with the appearance of goodness and being "fair" may actually be corrupt and "foul." Certainly Macbeth and Lady Macbeth reveals themselves to be duplicitous in their appearance of being kindly hostesses to the King Duncan while at the same time plotting his death. Lady Macbeth advises her husband to hide his treacherous thoughts:
"bear welcome in your eye,
Your hand, your tongue; look like the innocent flower,
But be the serpent under't" (I.v.69-71).
Lady Macbeth's suggestion reveals her own potential for deceit. She is definitely not what she outwardly portrays herself to be, which is a kindly, gracious woman. Like the witches' chant, Lady Macbeth appears to be "fair" but is inwardly "foul" (I.i.11).
Macbeth is a play about contradictions and the difficulty in perceiving people's true intentions and motives. The witches' chant of "Fair is foul, and foul is fair" becomes Shakespeare's own prophecy for Macbeth, predicting that the characters are not always what they pretend to be (I.i.11)