About knowing where invisible target are. It's actually how it should be by DnD rules. You don't know where "hidden" enemy is. Being "invisible" doesn't automatically make you "hidden". But I doubt that this was made intentionally.
Fairy fire on the other hand definitely shouldn't affect targets that succeed on saving throw.
Funniest thing about spell targeting and high ground is that it isn't simmetrical. I tried 2 situations. Targeting Astarion with sacred flame.
If he on the high ground you can't target him. But if he and Shadowhart swap places, game allows you to hit him with the same spell.