It seems like the original author was trying to turn the boyfriend into an ahole, while redeeming the group leader and maybe pushing a relationship between the leader and mc, typical of ntr.
Your writing has steered away from that. And although I'm a fan of poly, I don't think you're trying to get the best friend to join the relationship but instead have the boyfriend choose the mc over her, to make it seem like the boyfriend is the right choice.
But to me, I've realized that you and the original author have both written the mc to be someone who will be more and more sexually free and independent. This means the mc could be with someone who respects her sexual freedom, OR she could say f*** it and just not be in a relationship at all.
I think the best thing to do is make this a bit more cyoa. Date X or date no one, or date everyone. Be a housewife, broodmother, or ceo. Her future is open ended, and regardless of how you characterize everyone else, the story is still hers.