Yeah, it's definitely odd in retrospect. As you say, Sage's comment about her mom and Stephen's line about "I know you. You're not the type of guy to beat up a guy, unprovoked" suggests they knew about Sage and Chad. In that case, Sage's parents really should have recused themselves when Jill made allegations against their daughter's boyfriend - especially if they take it easy on Chad, as Stephen claims.
As for Chad, I don't get the sense he doesn't give a shit about Sage, I just think he's very, very deep in the closet. We don't know why their relationship started, but Sage
implies he agreed to it rather suddenly:
View attachment 1538276
If so, Chad probably found himself in a situation where he desperately needed some heterosexual street cred and took the opportunity Sage unintentionally handed him. Her parents had nothing to do with it and I doubt he liked treating her that way; presumably he broke up with her because enough time had passed that he didn't need to keep stringing her along.
Of course, he
should have told her the truth, but like Maya (for different reasons) he just can't bring himself to reveal such a profound secret even when it's obvious the truth will come out eventually.