I am curious to know, if I set up an event for OnSpellCast, then check GetCombatTarget, will the target reliably be the one the actor just cast at, even if they are technically in combat with multiple enemies?

This is for NPC's not the player.

Not sure what you are trying to do here, can you use IsSpellTarget instead?

Nope, I need to be able to check from the actor casting the spell who it is they are casting it at. IsSpellTarget forces me to check from the target.

I don't think so. May work most of the time, but a NPC could change it's target since the cast starts until the spell reach the objective. Keep also in mind that while getcombattarget returns the enemy the player is looking at, in the case of NPCs it's the enemy the NPC is engaged with, while it could be looking at another NPC.

Because of the script lag you will have to just work with "most of the time". Can you "double check" the target after it hits by having the spell itself set an actor quest ref to the target then your script with getcombattarget check this as well to see if they match?

Since OnSpellCast triggers immediately after the spell is cast, and not when it hits the target, it seems like as long as the actor only casts spells as its combat target, it is probably always going to work. I don't know if it's true that actors only cast spells at their combat targets though. You will have to test that. And of course it will not tell you who the spell actually hit unless it's a target actor spell, but you did not say you needed to know that, so...