What's the difference between these two entry points?

With Apply Combat Hit Spell, would there be some way of only letting it work with a specific hand? I.e., could I set up a "right hand perk" that applies my spell when the right hand hits and a "left hand perk" that applies my spell when the left hand hits?

In order:
1) Apply Combat Hit Spell makes it so that when you hit the target with a weapon attack, your weapon casts the specified spell at them.
2) Apply Weapon Swing Spell makes it so that when you hit the target with a weapon attack or a bash, your item grants them a perk that is Apply Combat Hit Spell with the specified spell. This is only used AFAIK to create the impact blocking effect of Quick Reflexes.
3) Yes, can do that with condition functions on the player category of the perk - GetEquippedItemType(0) is left, (1) is Right.

1) That's pretty self-explanatory, but...
2) I still don't quite understand this one. I hit a bandit and the BANDIT gets a perk that casts some spell back on me when the bandit hits me? Couldn't the exact same thing be achieved with 1) and having the spell cast add the perk to the bandit? I have no idea why this would ever be of use (I haven't checked out how Quick Reflexes works).
3) Thank you! I hadn't realized GetEquippedItemType could get the equip slot of the item.

Adding perks through Papyrus scripts is broken for anything other than adding perks to the player.. typing Bandit.AddPerk(somePerk) in papyrus will perfectly compile and all that, but it simply doesn't do anything in game.

I haven't seen if using spells as holders works for NPCs with perks, but generally I shy away from adding perks to NPCs from anything other than the actor category. They just usually don't play well with them.

Essentially, Quick Reflexes uses Apply Weapon Swing Spell to make sure that the player has both bashed and the enemy is attacking; it's very good for Impact Parry effects, basically.