I’ll grant you a spoiler
Question 1. Yes Ido regressed, this is the 2nd timeline.
Question 2. Yes this will have a happy ending, a very satisfying ending at that.
Question 3. Why Sejin can’t remember, it was not explained but when Ido followed Sejin to his death in the 2nd timeline he saw Sejin in his subconsciousness and there he said. If he could be given a 2nd chance in life where Sejin is with him. He wish to be the only one to remember so he can make things right. Then when he woke up he thought his suicide attempt was a failure but he felt Sejin and that’s when he realized he went back to the past months before his marriage. (This is a major spoiler: Ido forcedly onesidedly imprinted Sejin and in this omegaverse universe all imprinted couples shares feelings and memories.) Since Ido is one sidedly imprinted to Sejin then he’s the only one who can feel Sejin’s feelings more than anyone else and he has memories of Sejin’s from the past so that explains why he knows all Sejin’s preferences.
In this timeline they will get married again. It was an open ending in the novel. They both are trying to find healing together.
Can someone spoil me? Like I'm so curious about their past i just wanna know why the uke doesn't remember anything and his dreams confuse me too like is that the seme? Also someone said that the seme regressed is that true? Will it include a happy ending cuz the seme sounds like such a green flag now and i think our uke is forgiving enough yk? :3