I was in an open relationship (well, a hierarchical poly relationship, but it's basically the same thing) for over a decade. Honestly, it was great. I also learned a lot, so if there's anything specifically you're curious about I can probably give some advice. For now, let me address your concerns.
Depending on the precise nature of your fiance's rough upbringing, it may indeed be a major component on why this kind of lifestyle is attractive to her, but I would be very, very careful about attributing it to any kind of mental illness. People with rough childhood experiences sometimes don't experience the emotions of being in a relationship in quite the same way. That isn't mental illness and it can't be fixed with therapy (at least, not without many, many years of very difficult therapy) it's a kind of self-defence mechanism (called "dissociation") which children in that situation learn, and while it can occasionally cause problems in adulthood, it is a necessary part of how human minds work. Whether you think you need therapy is up to you, but I find the most important thing is to be willing to examine yourself so that you can have a clear idea of what you want.
Secondly, while there is no guarantee that any of your secondary partners will ever become "something more", it is important to remember that they are people with feelings and needs who enter into a sexual relationship with you or your partner to meet those needs. The kind of very clean relationship where you just have sex with other people but never feel any emotions towards them probably isn't going to be possible, because very few people are going to be entirely happy with playing unicorn like that. You can conceal it from them, of course, but that's a pretty abusive thing to do and raises the question of why you're doing this in the first place. So no, the chances are that eventually you will find you are emotionally involved with a sexual partner, or your fiance will. It needn't be in the sense of a conventional romantic relationship, but it doesn't need to be. The joy of living an unconventional lifestyle, I think, is that there are no "rules" in that regard. Still, it is a good idea to treat your other sexual partners like human beings and ensure that their needs are being met.
Also, if one or both of you does meet another partner whom they like and want to be emotionally involved with, then that doesn't have to mean treating them the same in terms of time and emotional investment. Some polyamorous people dislike the concept of hierarchies, but in my experience most successful poly relationships are hierarchical. The line between an open relationship and a poly relationship doesn't really exist, in this sense.
Several people have mentioned "time" in poly relationships, so I want to correct something here. This whole notion of time is predicated on the assumption that you're the only one in the relationship who has multiple partners and that those partners all expect the same amount of time from you. One of the great things about a poly relationship is that there isn't this intense pressure to always meet someone's needs. You do the things you're good at, or which you have time to do, and other people do the rest. Everyone gets their needs met, just by different people who are good at different things.
Anyway, on to the questions.
axlryder said:
Anyway, I shared all that so it might give you all something to chew on when considering the topic as a whole. What are your feelings? How do you feel about it "morally"? Would you ever do it? Have you done it? How did it go? etc.
1) Overwhelmingly positive, although in your specific case I worry that it may not be something you really want, but rather something you're tolerating. That's okay, I started out like that myself and it got better, but I'm not self-absorbed enough to assume everyone is like me.
2) Possessiveness towards human beings never really sat well with me, so personally I tend to see the ethical compass as favouring non-monogamy. That said, all forms of relationships can go bad or even become abusive, and non-monogamous relationships do provide opportunities to screw people over if you're not careful. I think that's something to be wary of and to take care over.
3) Yes.
4) Yes.
5) It was the best decision I ever made, and I don't regret any of it.