First of all, "real women" are broken. Everyone is "broken" in some way, so to say that Jill is the most "real" because she so far hasn't shown much signs that she's dealing with anything serious or has negative traits is ridiculous and it does not mean she is more "real" than the others who, in my view, are far more real because of their faults and insecurities. Jill is, at this moment, too perfect, and the hammer is eventually going to fall on what it is that makes her not so perfect as well, otherwise she just becomes a charicature of perceived female "perfection".
As far as her stance on relationships goes, what she says here seems, to me, to be ambiguous. It doesn't necessarily indicate monogamy as "being in a relationship" could mean a relationship with more than one person. "Seeing others" is therefore a broad, general term for seeing anyone outside the defined relationship, so if it were a committed relationship of 2 or even 3 people, going outiside of that to see someone else would be "despicable". She might not mean it that way, but then again, we don't really know for sure what her stance would be on a poly relationship, therefore this statement, until we find out otherwise, could be taken as a more general statement about stepping outside the bounds of an established relationship.
This actually ties in to the point about "moral fluidity", because until we know what her stance is on relationships, whether she believes in only monogamy or not, then her morals at this point are, from a speculative standpoint at least, fluid because we don't yet know what they are. Right now, she could really fall on either side of this and even if she was on the monogamy side, it doesn't necessarily mean she's rigidly on that side as certain cirsumstances coupled with well reasoned arguments in favour of it could sway her to the other side, particularly when they involve a close friend who she may have shared an intimate moment with.
Poly relationships may be rare, but it doesn't neceaarily mean they are "doomed". In any relationship, it's about having a foundation built on mutual trust, affection, and understanding, and as long as everyone in the relationship knows where they stand, then there's no reason it can't work as long as they each put in the effort to make it work. But, then again, finding actual stats and figures on poly relationships are almost impossible to come by due to societal stigma, so it can't be said for certain that such relationships can't work or aren't more common that we might think.
Monogamy is another one of those puritanical principles that, in some ways, actually goes against the "human condition". In fact, we've been "conditioned" to believe that only monogamy is acceptable which is what causes so many problems when it comes to people in committed relationships falling in love with someone else. In reality, men generally still desire multiple partners as it's part of the primitive/primal brain of being a dominant male and to propagate as much as possible, and women will often desire one man, much like it is in the animal kingdom with apes and lions. It is possible to love more than person in a romantic way, but society tells us we can't which, in essence, goes against nature.
Therefore, what matters most in a poly or even open relationship is the understanding of the situation. If 2 people enter into a monogamous relationship and one person cheats, then that is a betrayal of the terms of the relationship, but if someone enters into an open relationship then it is not a betrayal, and in a poly relationship the understanding is that at least one person has at least 2 other people they are intimate with. So such a relationship can be rationalised as long as all parties know the terms and are OK with them.
Again, we don't yet know Jill's stance on such a relationship. She may not disapprove of such a thing even if she thinks it's not for her, or she may never have considered the possibility for her but would have no problem with it if it happened. We really cant say for sure.
From a writing perspective, anything is possible and anything can work as long as it's written well. I admit that a Bella/Jill/MC poly or even sharing relationship is a long shot and I'm not saying that it is any way guaranteed, but I also don't see it as being totally outside the realms of possibility and plausibility.
For one thing, they can still be pursued somewhat separately by not pursuing Bella in favour of Jill, and not being particularly warm towards Jill in favour of being more interested in Bella, so it's not as if it would be forced like the M&J relationship. Secondly, the possibility that they once shared an intimate moment with each other could be a factor, indeed a "foundation", of setting something up with the 3 of them. And thirdly, whilst it could be seen as "too much" to have another such relationship possibility, it could also be seen as providing a greater level of choice for players since we have a M&J threesome route, possible M or J solo routes, a Sage route, a Jill route, and a Bella route, so a potential Bella/Jill route is simply providing another option, particularly for those who might want a poly relationship, but prefer B&J over M&J.
At the end of the day, whilst I acknowledge that this relationship might seem like something of a pipe dream and it is in no way guaranteed, it's not impossible nor would it, in my view, ruin anything with these characters, this story, or DPC as a writer.
