Could there be a clue that suggests the purpose of the orbs or is it not necessary because the party will infer that a key must be available somewhere in the dungeon? Perhaps touching the orbs initiates a scraping sound in the distance or a glow on a door visible from the room. These may be too simple but you get the idea.
I used to play with a group that played all sorts of OSR stuff and they would have loved it. The way you combine story telling with dungeon in 2 parts feels really unique to me and is not a gimmick, it’s cohesive and complementary. In four pages! Sersa Victory has an adventure that has a wild twist ending. Shrine of the Jaguar Princess . This is reminiscent, but yours also is a storytelling device. If you’re not familiar- his stuff is fantastic.
Maybe create challenges that bring out the character types you introduce? Can be very subtle and open ended.
My biggest suggestion would be to expand the dungeon quite a bit and continue to explore the themes of the journey before the black pool. I want more! Very evocative writing. Easy to run. Really dig it.