I like your theory! It makes a lot of sense and gives the readers a clear team. I would die inside if it was swapped, because I like the white haired goblin boy from the past life.
Plus their names being similar including the family name makes me think it’s a bloodline thing. Maybe they were bound together in a past life, which is why Shin Yooyoung was drawn to our mc. Then he gets hit by the truck and dies, then our white haired past life boy takes over. Those 2, the young master and goblin from past life, seemed to have something up. Might explain why the goblin was able to take over his body.
Hm what if his past husband was the dead shin yooyoung and 'our' shin yooyoung is the white haired boy (obviously)? Then that would explain why he hated og SYY sm bc he was abusive towards Kwon in their past life, maybe SYY did something to the guy out of anger and Kwon got implicated in it which explains why he's guilty towards Kwon. Maybe it even led to Kwon's death?
Its either that or they take the yandere route. He kills Kwon's husband and imprisons Kwon for himself. When Kwon dies he binds his soul or whatever so he can follow Kwon into the next life. Bc we still have the supernatural factor to take into account, and the fact that ML was previously called a 'curse' or 'goblin'. That means he might have some unusual abilities.