SureAI

I'm stuck on the "third wish" for this quest, as the door to the cave under the waterfall is locked and nothing happens when I bring Ryneus there. If I was told where a key is I don't remember it, the text in the quest journal is just giving me tips on catching butterflies.

He should have given the key to you if you are near that door. It is a known issue and already mentioned in the bugtracker.
You could unlock the door via the console command while you have clicked on it (showing the RefID): Enter the cave.
It could be that the questmarker directs you out of the gave but just walk in. It should trigger the quest.