How to reduce a skeleton's shoot speed and damage?

EntityAIAttackRangedBow#attackCooldown

controls the shooting speed of skeletons.

AbstractSkeleton#setCombatTask

is used to update this value when the skeleton is instantiated, read from NBT or spawned and when its main hand held item changes.

 

I'm not sure if there's a good event to modify this from, since

EntityJoinWorldEvent

can be fired either before or after

AbstractSkeleton#setCombatTask

is called from

AbstractSkeleton#onInitialSpawn

and

LivingSpawnEvent.SpecialSpawn 

will always be fired before it (and only for skeletons spawned passively or from a mob spawner).

LivingEquipmentChangeEvent

may work here.

 

Arrow damage is based on the

EntityArrow#damage

field (this is set from

EntityArrow#setEnchantmentEffectsFromEntity

for skeleton arrows) and its velocity. You can use

EntityJoinWorldEvent

to detect an arrow being spawned, get the shooter from the

EntityArrow#shootingEntity

field and set the damage with

EntityArrow#setDamage

.