I haven't figured out anything useful yet, but if it helps, when an NPC attacks, the following events seem to be working fine:
Events.OnWeaponHitCharacter
Events.OnWeaponSwingHitPoint
Events.OnPlayerAttackFinished
Hook.Attack
Events.OnWeaponSwing
The events fire normally, there is even blood splatter after some attacks, but nobody ever takes damage. This applies to both attacks initiated by NPCs as well as attacks done to them.