You redirect the damage to the attacker because...
"glossary" wrote:When this character or an adjacent character is hit by an attack from an enemy that isn't adjacent to either character, the attacker makes a save of 11. If the save fails, the targeted character takes no damage and the attacker takes damage equal to the prevented damage
It says the attacker takes damage equal to the prevented damage.
As for the stacking, I'll let someone else explain how that works, cause I'm not sure.