I am thinking that a way around having a potential-wife run off at the idea of dating a married man, is to in a social setting where the husband is with his wife, *they* befriend her, and over the course of a number of meetings, they show her that they're kind, sane, and Godly. If it can be arranged for her to meet the children and see them interact with them, and that they're well behaved and well adjusted; so much the better. At some point, the *wife* could drop some poly-positive related hint, either in a bible discussion or pop culture reference via Sister Wives or the like. The point of the wife doing it is to eliminate any idea that the husband is some sort of sex maniac, or going behind his wife's back, or he is the one pushing for it. It also serves to draw the woman into the idea via herd mentality seeing a peer (the wife) interested. She could drop a hint like this; in a dreamy sort of way, comment how that it sounds so nice to have a close friend like that right there in the house.
Be very careful with the hint that is dropped so that if she is utterly repulsed by the idea, you've not revealed too much and then she report you for committing a thought-crime. If she seems not repulsed, but casually supposes it wrong (...but isn't polygamy wrong?), you can matter-of-factly/casually comment how heroes of faith did it, the witness God bore of Abimelech in Gen 20, and the witness God bore of the law-abidingness of Abraham the polygynist in Gen 26:5, and that polygynists go to heaven (Luk 13:28), and that nowhere is poly spoken against, and how it was an important social construct to see that women were cared for. Note how that even today in Ukraine, the women outnumber men roughly 5:4, so 20% of the women have not a warm body to marry, let alone one who can be a good husband. Other hints that could be dropped to explore her psyche are things related to patriarchy, submission, and anti-feminism. It seems good to proceed in such a away as to not make an enemy if it turns out that she seems not compatible.
Concerning bible discussion and submission, I am a big fan of the idea of the husband leading bible studies with the family in the home on Shabbat. With that in mind, he might casually start a bible discussion which was pre-thought-out to lead over top of some poly in the bible. Then, the wife make a pre-thought-out poly-positive remark. As for the aforementioned submission, the frame here is the husband leading the family, and the women following along; even the potential sister wife. I believe that God ordered things this way, and for women to be content and thrive in such a environment, and so presenting the God-ordained situation to her, her innate nature might kick in and cause her to be more amenable to the process. I like the idea of "Begin as you mean to go on"; so as much as is socially acceptable, treat her as a wife/sisterwife right from the start, and see if she settles into that role.
Having the wife along and this showing that another woman is interested in him, and this giving him preselection points (proven quality) with a potential wife, is a very interesting thing. Yeshua commented "everyone who has, more shall be given, and he will have an abundance" (Mat 25:29). Early on I couldn't make sense of that, but now I see it as a comment on how the world works. We see it with women being attracted to a preselected man. So in that way, attracting a second wife can be easier than attracting a first wife. I see it in my own life, it's the snowball effect. As I gather more resources, I can do more things, more effectively, and faster; and this allows me to gather even more resources, and the process accelerates.
Now for the sobriety..
I think a significant component here is charisma and leadership from the husband. Women tend to be very leadable, and are sadly oft led into all kinds of troublesome things by charismatic men doing things they ought not. I think that strong leadership on the husband's part can lead her in the way of Godliness and away from worldly filth (monogamy only). Though, I see a power struggle, an authority contest; who ultimately holds more sway over her? Can she stay stepped out of the world when and if her friends and family pressure and forsake her? If not, trying to build a poly relationship with her seems like a disaster waiting to happen. Even if he holds more sway over her than those pressuring her in the world, it is still likely to be a source of stress for her, and thus the family. It might eventually wear her down, and then disaster. So unless she can disregard their pressurings, or they support her; it seems very risky to pursue a poly relationship with her.