QIf I have 2 copies of this follower on my field and both of their abilities take effect, how many times is damage prevented to my leader?

A2 times. Since the first instance of damage would be prevented, it would not trigger the second instance of this follower's ability, which would persist until your leader would take damage again.

QIf this follower is on my field and my opponent has a Destruction in Black on the field, in what order are the abilities resolved at the start of my opponent's main phase?

AAt the start of the opponent's main phase, both this follower and Destruction in Black's abilities would trigger and become pending. The turn player's pending abilities resolve first, and Destruction in Black would deal 2 damage to your leader before you resolve this follower's ability.

QIf a follower with 0 attack, such as Bellringer Angel, attacks my leader, does it use up this follower's effect?

ANo. Attacks made by followers with 0 or less attack do not count as having dealt damage, so this follower's ability would remain in effect.

QCan my opponent use a follower's ability, such as Azazel's On Evolve ability, to change my leader's defense while this follower's ability is in effect?

AYes. Modifying defense isn't the same as dealing damage.