Seonwoo I's part happened simultaneously with Seonwoo II, they just should that at the end to demonstrate Seonwoo II obtaining the Keymastar's power, which is I suppose the observer of dimensions. At the end Seonwoo II's clok going backwords indicated Jehee using his regression ability. And II and Jehee met again back in time and Jehee changed his approach in meeting II as he had once mentioned before.

***Question regarding the ending ***
So, Yeonwoo I is a version that got swepped into the game 20 years ago and worked as the clinc doctor. Yeonwoo II got into the game later and took over the live of Yeonwoo I ingame, when he came over as an adult. When Yeonwoo II died ingame, Yeonwoo I came back into reality and when Jahee regressed, Yeonwoo II lost his memories about the ingame romance etc and came back into the game...? Did I understand that correctly?