The situation about Sage is really annoying although I get it what was the idea behind it.
With all LIs you've got an emotional connection first and then physical and sexual. Sage is supposed to go in the opposite direction - she'll have sex with MC and everything but she'll become emotionally interested in him as time progresses and when she'll saw the foolishness of spying Chad.
About other girls... A long time ago when I played my first VN - I think it was "True Love" or "Season of Sakura", both really nice dating sims - there was one specific path. You could basically choose which girl to pursue but you could also choose not to bother with any. If you did that, you'd find a little cat. Of course, that's Japanese game so it's not just a cat but typical neko-girl.
I don't see why here wouldn't be possible to do the same thing. If your MC was man-whore fucking everything alive, there could be special ending for him. Also, if your MC didn't pursue any of the main girls but just scenes with one of the side girls, it could have another ending. It's quite simple for the coder, really.