1. I agree with Kirk, Leviathan does not get SI to interrupt OoN.
2. Because StZ specifies opponents' cards, the only cards that are protected against are ones that are owned and controlled by someone other than you. Since KM is owned by you StZ doesn't protect from him.
3. browarod is correct. Just like Thaddeus protects decks from being drawn from from abilities used by the appropriate ECs, Simon protects against forced draw. Fortunately he also protects your hand, so that Mayhem doesn't make him unplayable...