Fujihara Tomomi:
I feel like Jang Uk and Bu-Yeon are born with the Hwansu-level energy already. This can explain why Bu-Yeon was so powerful even as a child. Since Jang Uk's energy gate was blocked, he couldn't bring out his full potential and would take some time to reawaken his power like Master Lee said. They are both born from Ice Stone anyway, so it's like their birthright already.
Yes, the Queen is from the Seo family, Seo Yul's father's sister, which makes them cousins. Jang Uk is the former King's son, who was the older brother of the current king. So yeah, that makes them cousins too.
I dont rember who said it, but the person said that is only 2 person that have reach Hwansu. One it ofc master Seo Gyeon and they didn't mention the other person. The next person that's are gonna reach it is Uk(ofc he is the hero). base on ep 9 Jang Gang haven't reach Hwansu yet, maybe I have reach it and decided to return.
My wild theory is that Jang gang are gonna teach Uk before he die! Why I think Jang Gang have to die is bc you can not have a line up with Jang Gang, Master Lee, Park Jin, Yul and Uk vs Shaman Choi/the Queen and Jin Mu. Its too easy for the "good side".