Basically he staged shinwoo's death with help of his older brother, which gives him some time to find out something against his dad to blackmail him to not force marriage onto him. He returned to Office to find out a dirty secret about his father past and found that letter. I think the letter says something about the incident with an interpreter (might be Shinwoo's father) who died i'm prison for something that we don't know yet and how all the testimonies were dismissed and the case closed. I hope You can understand (sry English is not my first language)
I think it has been hinted quite a few times throughout the story so far. Hyowon actually mentioned that the noble lady that was his crush when he was younger looks a lot like Shinwoo. The noble woman was actually Shinwoo’s mother. Not sure why Hyowon’s father killed Shinwoo’s father even though he knew Shinwoo’s father was innocent. This is what I have put together from the hints so far.

So im lazy to re read the chapters, but what was on the letter that hyowon found and why is he hesitant to be with shinwoo? ( ̄へ ̄)I missed something