It could be this way: What Buyeon and Uk have within them is the energy from the ice stone, not the ice stone itself. The constellation basin is something like a medium to connect with the skies. So when Buyeon’s hand was dipped in it, the skies sensed the ice stone’s energy in her, which resulted to some sort of reaction (the thunderbolt). But why thunder tho? Do the skies want the ice stone back?
The ceiling was opened not because she’s a constellation mage, but because of the ice stone’s energy Since the ice stone is the child (lack of better term) of the skies. Also, if the ceiling really opened because she’s a constellation mage, then the constellation mages handle the scary black tornado in the sky whenever they perform their job. I doubt it.
The ice stone is a product of the skies. It’s a hail that cannot be melted. Then, the rain and lake waters carry the same powers. It just that the powers of the ice stone is more concentrated because it’s compacted in a ball. Therefore, the rain and lake waters enhance the innate powers of Buyeon and Uk.
Master Lee saying that Cho Chung’s daughter (the show implied it‘s Naksu through “obvious” clues) is dead could be a hidden fact. Besides, Master Lee knows it all. That can be an evidence that there are no two souls in one body. And he checked Mudeok’s body on their first encounter. Does that mean he didn’t see Cho Yeong’s soul in there?
Also, Seo Gyeong is the founder of Songrim, but why is Park Jin the leader now? If it’s about hierarchy, it should be Seo Yul’s father or at least Heo Yeom since the former is unavailable. But why is the Seo family managing the Sejukwon, which is just a subsidiary of the main institution? Was the Seo’s demoted?