1. Why do you believe what you believe? What would change your mind?

My beliefs are based on a combination of experience and values.  In my opinion, the ultimate goal of a relationship is marriage.  Not the legal sense, but in the full “two people making a lifetime commitment to become a single unit based on their love for each other.”  I have been lucky to grow up around many marriages like this, and I’ve made it a goal for my own life.  That kind of marriage is a powerful thing, and a society made up of marriages like that would, I believe, have superior outcomes to any other society.  That kind of marriage also takes work, years and years of work, but the greatest rewards require the greatest investments.I understand that many people may not value the same things that I value, but to me choosing to pursue relationships with no long-term potential is similar to choosing to live in your parents basement and play videogames all day, instead of getting a job and supporting unhealthy for the individual and the society that they belong to.  This is a case of “the whole is greater than the sum of its parts.” Poly relationships…just seem like parts.

A consistent pattern of poly marriages lasting 20 years or more would make me change my mind. I have never heard of a poly group that was truly long-term in the sense of being able to raise a child.  Obviously a large number of vanilla heterosexual marriages fail too for all kinds of reasons, but as far as I can tell the failure rate for poly relationships is 100%.

2a. A monogamous person is jealous of their partner (for example, because they’re afraid their partner has a romantic interest in someone else). In a healthy relationship, what would happen next?
In some situations jealousy would be a valid response, e.g. if their partner was actually interested or having an affair with someone else. But since you specified a healthy relationship, we can assume there is no basis for the jealousy. Therefore, the next thing would be for the jealous partner to communicate their concerns, and together find a way to address them.
2b. A monogamous person has a crush on someone other than their partner. In a healthy relationship, what would happen next?

That person takes steps to distance themself from their crush.  Like the old story about “two wolves in side of you” the feelings fade away if they are not fed.
3. What would happen if 90% of people in a society were polyamorous? (You may assume they all practice one style of polyamory, or different styles.)

The notion of exclusivity in a relationship would disappear, followed by the notion of commitment. Relationships would become ephemeral–instead  of having partners for life, people would just have a list of whomever they happened to be sleeping with at any given time.  Additionally, without the focus on monogamy and commitment, there would be little appreciation for the work that goes into maintaining a healthy relationship.  Instead of encouraging one’s partner to grow and address their defects, the incentivized behavior would be to simply find a new partner.  Such a society of adults may be able to function well enough, but such a society could not provide a consistent environment for children to grow up in, and as a result a society like that could have no future.