QIf this follower fights an enemy follower and both are destroyed, do I deal 1 damage to the enemy follower's leader and give my leader +1 defense?

AYes, you do.

QIf this follower fights with an enemy follower and both followers are destroyed due to combat damage, can I still play this follower's ability?

AYes. Even if both followers are destroyed by combat damage, the ability's condition, "whenever an enemy follower is destroyed," is met.

QIf I use this follower to attack an enemy follower with 5 attack, and both followers are destroyed by combat damage, does this follower's "Whenever an enemy follower is destroyed, deal 1 damage to its leader and give your leader +1 defense" ability trigger?

AYes, it does.

QIf I have 2 copies of this follower on my field, and an enemy follower is destroyed, does each copy's "Whenever an enemy follower is destroyed, deal 1 damage to its leader and give your leader +1 defense" ability trigger?

AYes. Both abilities will trigger.