I don't think that's what happened.
When Seungwon was leaving just after having talked to Seungho, we see Seungho's servant giving him a rolled paper. We didn't know what it was until now... Seungho is the one who sent his father the drawing. That shows how much bad blood there is between them. Seungho will not go visit his father even if he is sick and on top of that, he makes sure to give him the finger with that drawing.
I don't think Seungwon has any idea of what he was delivering. He probably thought it was a letter from Seungho and he didn't opened it.
I think Seungwon is as innocent and good natured as he looks... the best theory I see around that may become true is of those who think that Inhun will ingratiate himself as his friend and probably pry on Seungho's family secrets through his younger naive brother.
Oh Thanks for the long reply. That's another good idea.
In.Ch.35 SeungHo didn't even read his father's letter, so I think he didn't want to involve with them anymore. If he wanted to give SeungWon anything, he could have just handed him directly. That's the painting Jihwa destroyed by pouring water on it, so it's possibly thrown away and someone kept it.
I more concern that InHun tries to blackmail them, investigated about the treason, how his father strangely retired from government then moved to live in a secluded area.
Now he also already realized Na-kyum is SeungHo's weakness.
SeungWon disappointed me. In Ch.37 He acted like helping father & brother reconcile, but in Ch.44 he just threw fuel into fire by handing father pic of SeungHo's threesome.
So both Jihwa & SeungWon have planted spies in SeungHo's house. #－.－)
The green-clothes guy way reacted to SeungHo in Ch.41, makes me feel that he tried to kill 2 birds with 1 stone to get SeungHo by manipulating Jihwa to murder Na-kyum. I may be wrong tho.