I think that uncovered first life story is something different from the novel -
however SM should be real son of Duke and Madam Jiang... but his father never had feelings for his mother and he was greatly supressed by late Duke - as he wasn't a person of great character or capabilities - that's why Duke engaged him to one of Jiang family girls (well known generals family for its fertility and discipline training) going against his son wishes. Duke had a mistress which became pregnant at the same time as young madam Jiang and when his affairs had been discovered by late duke - the girl fed abortion pill and let to die - which old duke people thought happened. After whole story he has been supressed even more by late duke and his wife... few years later he found out that his mistress survived and Song Han was actually their second baby. Then we had newborn babies swap - so Madam's Jiang daughter had be left with mistress family and boy (from forced early labour) taken into duke mansion.
Years later when Madam Jiang found out about the mistress and "her" daughter and offered to bring them both to the Duke's mansion as well as recognition for the child - Duke decided to kill his wife - as otherwise she would understand immediately that girl was her real daughter and she was raising up mistress son.
She had been fed a poison together with the medicine . Song Han already knew truth about his origins and when discovered that Duke is poisoning Madam Jiang did nothing - and was still boiling and giving medicine to "mother". When his real mother started to black mail him - he send his servant to kill her... Like a father like a son...
So when Song Mo found out the truth in the first life timeline - he had killed both father and half brother...
