QIf this follower fights an enemy follower with 0 attack, do I still give it +2/+2?

ANo. Followers with 0 attack don't deal attack or combat damage, so this follower's ability doesn't trigger.

QWhat happens if this follower takes damage that brings it to 0 defense or less?

ATaking damage triggers this follower's ability, but by the time you resolve it, this follower has already been destroyed and put into the cemetery, so you don't give it +2/+2.