If you were trying to sway someone to consider an alternative way of dealing with insurance, inheritance, power of attorney, etc. what would you propose?

For health insurance, well, the policy can have its own procedure to add new covered members, like car insurance does now.

Power of attorney already has its own mechanisms for transmission.

For inheritance you can write a will, of course. Spousal transfer is just the default, and there's usually no confusion even in weird situations like common law marriage. In many cases there are children with obvious partial claims.

Married people shouldn't get tax breaks, if anything multiple-wage-earning households should have higher taxes, and tax breaks should be based on the number of non-income-generating people in the household.

one thing that seemed to come up was jealousy, which I can understand. From your perspective and experience with polyamory, how does a person deal with it?

"Don't be jealous" is the optimal solution. I guess that's not very helpful but it really is the best course of action. It helps if you have good personal relations with the other partner(s). Healthy poly relationships depend heavily on openness, communication, and mutual support, just like any other relationship.