Does he cheat a second time later in the story?
I don't think he did again, the thing in the extras (the novel) was someone trying to sabotage Shao Qun. After the first time and everything that happened he just went full obsessed mode but he doomed himself to walk on eggshells the rest of his life afraid of Chengxiu leaving him, that's what it looks like in the rest of the extras years later when they live together.
Everyone's complaining that both semes in this story are horrible. Not everyone has a happy story. People date abusive person after abusive person in real life to. Of course we want to see him escape and be free, and yes the seme is deranged. It's painful to read, but we can't have a knight in shining armor and happy ever afters everytime. Stories that are realistic can be pretty gruesome and bitter.
But the ending was rushed and lots of loose ends not tied up. I still enjoyed it and it's a good historical yaoi read.