So, I have a quest. It's already been started, and it's currently functioning.

I want the quest to automatically add a spell to my character (I've made a new functionality), but OnInit doesn't seem to work. Is it because the quest is already started? How do I add the spell to my character then?

put the addspell code in a quest stage, and use an external object to advance it to that stage.

if you cant use stages for whatever reason, you could run a parallel quest dedicated just to "control" your already running one. place the addspell in a function in the quest script and use the controll quest to call the function

Isn't it, like, overkill to create another quest just for that? But I guess my option is limited. The quest is really just an excuse to add InventoryEventFilter and OnItemAdded event

Also, a question: If I load a save that haven't had this quest before, will OnInit fire and add the spell?

I'm thinking of deactivating the mod, saving, and reactivate it again. I've heard of the "script persists" stuff, but if I reactivate the quest, will the "persisted" script be replaced or still laying there dormant?

yea it should.

also in vanilla skyrim there are tons of quests that are solely for controlling other quests. its really the only way to non-sequentially leapfrog through stages, as a quest cannot set itself to a lower stage

When you say "yeah", are you talking about "brand new save will trigger on init" or "deactivating and reactivating should work"?

You can always load an old save from before you activated the mod the first time. That will cause everything to initialize again