Boundaries are actually good!
The common denominator I see in poly relationships is poor boundaries, regardless of how healthy the poly relationship supposedly is. Polyamory also seems to attract a high percentage of self-absorbed narcissists, like the OP's partner Carol and the metamour Maggie.
Obviously, not everyone is this way, etc, etc. However, so many people seem to go into poly arrangements with the naive belief that they can have an enlightened relationship, that everything can work out if everyone is reasonable. That people can be giving and generous with each other out of the goodness of their hearts, even in situations where they have to put aside their own romantic needs.
This is a great fantasy and it seems to attract people who have high trust in others, and who pride themselves on their empathy and compassion. However, it also attracts people who will take advantage of this level of pro-sociality and ignores how humans actually behave in the real world. In the real world, there is jealousy, envy, emotional power plays, passive aggression, manipulation, and resentment, all of which will amplify any problem within a poly relationship.
This is not to say that I think poly relationships are inherently bad. But when there are so many people with competing needs, it is just inevitable that a hierarchy in the polycule is going to form. Someone will feel left out and ostracized, like the OP. It is especially notable that her partner Carol seemed to be totally ambivalent and dismissive towards OP in increasingly cruel ways. Carol clearly was too cowardly and passive aggressive to initiate a break up, probably because she wanted OP to continue contributing to the rent.
Poly relationships can only work if there is a certain emotional detachment between partners. If you are the sort who really values stability and "devotion" from your partner(s) --as the OP indicates in the article --polyamory is going to be a challenge for you. It is just too easy for someone with two romantic partners to start prioritizing the one who is less emotionally needy, or annoying, or simply less hot.
The other issue with polyamory is that there is no playbook for poly relationships in the West. It isn't normalized, so we have no etiquette or social expectations for how a poly relationship should work. Relationships can really only thrive if there is external community support.