Well, I was playing around and there are some weird things afoot:
Tav (Wizard): Always uses Intelligence, be they cleric or wizard scrolls.
Shadowheart: Always uses Wisdom, be they cleric scrolls or wizard scrolls.
Astarion: Always uses Intelligence, be they cleric or wizard scrolls.
Lae'zal: Doesn't use anything, since she has no bonuses, I guess.
So, maybe if you have a 'casting stat', like Shadowheart, it defaults to that, if not it uses Intelligence?
Oh, okay. And anyway, if Astarion were to have a casting stat, ie was made an AT, then it would be INT. If your hypothesis in your last sentence were correct then at least characters would be able to use scrolls correctly.
In this case, I’d still prefer Larian to limit scroll use to class-appropriate spells, especially as I don’t think there’s a quick and easy way to check in-game whether a scroll is for a spell on a character class’s list, but it would be slightly lower priority for me as at least I’d have the option to limit myself and get 5e-like behaviour.
