I can't remember what happened to the ex but at the end, there's a board meeting where Kim is going to take over the dream project and the company. However, the ceo (yoon jay) has a secret trick, he owns the rights to the song used in the book which the dream project is based off. In the contract for the dream project, it is essential that, that song is used only requested by the author. So Kim basically can't take over the project. He then later "dies", yoon jay secretly murders him in an unsuspicious way.
The wattpad one got removed sadly and annoyingly because they had the side stories. I think someone finished translating the whole payback on reddit and there's the link above (idk if they finished but have a good translation at least). If you're truly desperate for side stories, hunt for the Indonesian translation on wattpad and google translate but it will be like gopplegoop to understand
I've read the novel and it's amaaaaaazing!! Y'all are just so tasteless and horny af~
Y'all should go read some other yaois with lots of segs scene and no plot ┐(´д`)┌