Possible interesting theory about Sage's relationship route, but what if, even after the break-up, she continues to mope and beat herself up over it, never fully recovering from it. On the FWB route, she and the MC maintain this status and grow closer, the MC even thinks they might start formally dating, but she resists going that far. She likes him, but she's been scarred by her relationship with Chad that she's afraid to go further with the MC than just FWB. She also knows that she's in her final year and likely to move away from B&R at the end of the academic year, so she feels like she doesn't want to start anything because of this, as it would either mean a long distance relationship or breaking up so they don't have to go through that.
Could this be how her relationship route goes? Because, let's face, this would be painful to have to deal with, having a girl you really like but who's now afraid of committment, and this seems like it would be right up DPC's alley to do this by not making it as easy as, "boyfriend gone, now she's mine". Maybe the MC might have to speak to Chad and try to get him to tell Sage the truth so she can move on, which is where sticking up for him in Ep 3 could come into play, and perhaps there may be a decision to stay at B&R or go with Sage when she graduates.