3 Meaning Hajin, Hanwool, and Hangyum?
Hanwool never (or rarely) switched with Hangyum. I think never for how Hanwool is acting (Chapter 21) . Hanwool never meet Hajin.
(Chapter 21) Hangyum is the former friend of Hajin. (Chapter 19, 21, and plus hints through the chapters)
I don’t think they were boyfriends or lovers in the past. (Chapter 8-9)
Does this help?
Oh I don’t know why Hangyum wants to hide his identity from Hajin. Because he does not want to lose him (chapter 19) but I think there is more to that.

so far I love the story, except the "twin brother" BS stuff.