When it comes to a court of law the onus is indeed on the accuser. But to successfully argue our position to the average man or woman down the street the onus falls back on us and if we ever want to be accepted not just in a court of law but in the community, then our arguments need to be both scriptural, solid and unbeatable before the average man and woman. Once that happens then the courts will follow.
I appreciate your zeal and understand what you're trying to do. I just think you're barking up the wrong tree.
For the record, I've been doing this 20 years and have had 100s of these conversations. I'm not trying to convince the average man or woman down the street of anything. And I'm accepted in my community.
Frankly, I'm not trying to win an argument with anyone. Couldn't care less. The ones with ears to hear will hear; the ones without, won't.
And my speech and my preaching was not with enticing words of man's wisdom, but in demonstration of the Spirit and of power: that your faith should not stand in the wisdom of men, but in the power of God. (1 Co 2:4-5)
The logic of Christian plural marriage is ascertainable by anyone with an open mind within a couple of hours. It's not a sophisticated argument; it's an exercise in pointing out the emperor very obviously isn't wearing any clothes. You either get it or you don't.
Beyond that, the best argument
for Christian plural marriage is
a good example. Live the life, show them what it looks like, and let the Spirit do the rest. Often, it takes time. Lots of time.
Jesus said, "By this will all men know that you are my disciples, by the eloquence of your rhetoric." Oops, my bad, that's not what he said at all....
A loving plural family is the best argument we have. A shipwrecked plural family is a counter-argument. It's the
doing that's going to influence people, not the talking.
Be the love. Be the incarnation of the Spirit of Christ. And sure, answer the questions you're asked to the best of your ability (1 Pe 3:15). Just don't frame this as an argument that we need to be prepared to have (and
win) with everyone we meet. That's not how this works.