Keep in mind, however, that the Paladin can only activate his Challenge once per round--once the enemy makes one attack that doesn't include the Paladin and takes damage, he's free to make any other attacks until the Paladin takes another turn. He still suffers a -2 penalty to all attacks that don't include the Paladin, but he's at no further risk of damage.
I don't think so. You can only Divinly challenge one enemy per turn. However, the text says nothing about you having to activate the divine challenge as an interrupt (only one per round).
PHB pg91 said:Also, it takes radiant damage equal to 3 + your Charisma modifier the first time it makes an attack that doesn’t include you as a target before the start of your next turn.