I have the same feeling that the ML regressed! Or maybe Seijin had an accident or something like that and lost some memories because its seems like they know each other from before, and the Ido's older brother speak like he knows him too, and I don't think they both regressed..
Damn i read the novel and you got most of the shit good job
You are 30% right in your speculation….
Spoiler below be warned…
1st timeline
It was a contract marriage true but it was Ido who agreed to it. Sejin’s father came to Ido with a proposal to wed his family’s Omega to Ido. Ido agreed because he doesn’t want to be pestered about marriage by his family plus since Heashin is already in the red he plans to absorb the company.
Ido doesn’t really care about Sejin but Sejin’s genuinity got Ido interested. Specially when his niece visited and Sejin spent time with her. (Remember the scribble in the poet book?) Sejin was the one who translated it in the first timeline and in the second timeline Ido wrote the translation and it was Sejin who was Ido pertaining to as his french teacher because in the 1st timeline Ido asked Sejin if he can teach him. They even shared their first kiss at the study table when Sejin was teaching Ido. Ido really fell in love with Sejin in the first timeline. When Sejin got framed up by his own father to embezzlement and tax evasion case it was Ido who got him out. Things got messed up when Sejin’s stole the file, Ido asked him for an excuse but Sejin didn’t give him any. Ido thought that Sejin never really loved him so Ido starts becoming abusive towards him. Ido brought Sejin’s family down after they framed up Sejin with the help of Director Kim.
Hmmm, not really certain but I think the ML regressed. Their first marriage was probably a contract marriage and Ido thought Sejin was doing the marriage and seducing him to take advantage of him and his family's company to make Sejin's family's company more successful. So he probably disliked Sejin and did not treat him well. There must've been a lot of misunderstandings and Ido must've been an asshole to Sejin and only realized his mistakes after he lost Sejin.
I have a feeling that the Ido was also the cause of Sejin's family company to collapse after he regressed, so he would have an advantage when asking for Sejin's hand in marriage this time. He looks like he wanted to make it up to Sejin by taking him away from his abusive adoptive family and giving him the life he did not give Sejin before.
I also think Sejin was assaulted by Ido's older brother, hence his reaction when they met. Because Sejin's dreams are memories of a life he experienced before but not in the current timeline. Although, some of those dreams seem to be of him and Ido, but when he dreamt about a stranger raping him, I think it was Ido's brother.
Also, this is a sad thought but I think Sejin took his own life using Ido's gun or was probably shot by someone else using said gun and died that's why Ido threw out all the bullets and told Sejin not to come to the study room. Sejin probably died there.
Well, these are my own speculations but I'm curious if any of these will turn out to be true. Either way, I am preparing myself for the coming angst of the backstory.