If you think about this just boils down to "I reincarnated in another world,now my ex is the femcel demon lord????"
Does she even qualify as a "femcel" if she can, in fact, get any guy she plots on, regardless of if she commits to the act or not? That's kind of a pre-requisite to be a party crasher, no?
That said, in a previous chapter, one of the heroines admitted that the reason they didn't want to believe MC was a genuinely good person, was because it was prove false their claim that "all" men are bad, and if that is indeed false, then that means they themselves either consistently got unlucky (
like consistently drawing a Nat 1), or got screwed over by men so severely that one time was enough to solidify the impression, which is a bitter pill to swallow for probably most of them but Koyuki (
she is a special case, giggity).
Healer girl got sacrificed for her party and got replaced. I forgot what happened with succubus/demon girl, but I'm going to guess her talent for magic earned her spite and prejudice from her male peers, and so on, including the noble chick. I previously speculated that the demon queen got killed in the same way, if not worse way than the MC by the same group of dudes.