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

ANo. This spell's effect applies "the next time your leader would take damage this turn." Followers with 0 attack don't deal attack damage, so the effect remains on your leader after the attack.

QIf I play this spell when my leader has at least 11 defense, then my opponent plays Azazel's On Evolve ability, does my leader's defense still become 10?

AYes. Azazel's On Evolve ability doesn't deal damage.