Guard is dispelled by any enemy action against the wearer (damaging or not);
With a Guard weapon equipped, Guard bonus becomes (M)DEF: 150%, plus reflected damage upon any enemy action;
Sleep is not dispelled by 'Steal' and can be used to rob an enemy of all it's possessions. Neither dispelled by an attack that happens to deal no damage to or heal its target (due to null/negative elemental resistance);
A confused character's attack is always random and will not be preferentially redirected toward an ally with Guard status. However, if a confused character attacks an ally with Guard "PLUS" status active, he/she will take reflected damage.
Buffs and Nerfs follow the rule of "Last one wins" i.e., if you are slowed, if you cast Quicken, you are SPD UP.
(*)Important : There seems to be a glitch regarding the DEF/MDEF DOWN nerfs; in fact, they appear to have no effect at all! Neither on enemies nor on heroes.