While I'm not that interested in what Chad's true situation is, you are neglecting a few things. Namely DPC likes to put in red herrings and convoluted if not nonsensical plot twists for the sake of melodrama. For example take Maya's big secret which was so underwhelming I was scratching my head why it was supposed to be a secret in the first place. Honestly I suspect Bella's story will be a similar let down after building up all that mystery.
Him doing it before doesn't mean he will do it in every possible moment. Everyone believes there's something bigger going on with every single plot point. This is not a complex psychological thriller movie, mind you. We already have heavy plots going on with Burke and Quinn. Because they have something to add to the story, so it makes sense for them to be kinda complex and not explained in every way.
But "Chad side bitch" plot is completed. Chad dumped Sage, it's over. Whoever or whatever it is do not have any effect on MC's story. It'll be Melanie.
Now, she can be a simple side bitch, or Chad actually likes her. Must probably the latter tho. If it was just a side bitch, he would still show at least sexual interest in Sage. But he doesn't do anything. It's not about Sage's attitude too, it's completely about Chad.
But for Melanie, it can be a simple fling. Maybe she doesn't have the same level of interest Chad has, but she might be just afraid of Sage discovering it's her. And she might be sad because she knows and saw how bad she fucked Sage up with it.
My point is, no need to go Sherlock over every single plot. I know waiting time is long and theorizing about everything is inevitable (especially some people here makes it so). But this is the reason of many points feeling underwhelming.
