Q. How should a plural wife guide her husband since he is her master? What kind of tone/method should she use? If she sees that he is making a mistake and he doesn't realise it, how should she guide him without undermining his headship?
If a husband is in error and his wife is in the right then she should attempt to minister to him using the inspired council of Paul in the case of a believing wife ministering to an unbelieving husband (1 Corinthians 7:13ff), even though this is only a circumstantial case of 'unbelief', that is, related to a particular situation or issue.
To begin with, Paul says:
The first key, then, is respect. The word "respect" means to honour or hold a high opinion of or have high regard for. Therefore a wife should speak as she would to a monarch, not raising her voice in anger, irritation or impatience, or demanding that he fulfil her wishes, but rather treat him with consideration politely and kindly. This is not a question of psychology (of appeasing his supposed maccho male ego or of manipulating him with subtle, wiley, catty, seductive female speech) but a question of recognising his divine office or calling as functional head of the family and marriage. This is not to say he is error-free (which he is not) but it is to separate the office from the man. As a human he is no different from the wife, anymore than a Pastor is from his congregation. Yet a Pastor should be treated with respect because of his office and calling because that is the calling which Yahweh has given him. To oppose either a Pastor or a husband's calling to be a functional head is to oppose Yahweh, and to undermine that calling by treating him rudely or disrespectfully is to undermine godly authority in the family. This may be said to be the 'legal' (letter-of-the-law) side of your question.
"let each one of you (men) love his wife as himself, and let the wife see that she RESPECTS her husband" (Ephesians5:33, RSV).
Next comes the spiritual, which forms a divine partnership with the legal. There is much practical advice that could be given:
If the husband is a Christian/Messianic and walking in the Spirit, then the best remedy for his error is to be taught by the Spirit through the gentle wisdom of a caring wife. Let the Spirit do the convicting of error, for this will have the most lasting and upbuilding effect since it will be Yahweh doing the correcting - the wife thus becomes His instrument. But if it is 'she' who is doing the correcting, then it boils down to a battle of egos, which is contrary to the way of holiness. If the husband is not a Christian/Messianic then totally different advice is needed which is beyond the scope of this ministry. If he is a Christian/Messianic and not walking in the Spirit then there will likely be a whole constellation of problems to be worked out and he becomes, as I suggested above, a kind of 'unbeliever'. If this is the sad reality, then the wife may expect to take some time in pointing him on the right path and it may well be that she may not have any effect in trying to persuade him by words. In such an advanced state of spiritual atrophy on the part of the husband it may well be that she can do little more than radiate the love of Christ silently and thus put him to shame by her godliness, thus leading him to repentance.
"A soft answer turneth away wrath, but a harsh word stirs up anger. The tongue of the wise dispenses knowledge, but the mouths of fools pour out folly" (Prov.15:1-2, RSV).
I do not think there is any one solve-it-all 'formula' for the resolution of such a question because so many different scenarious are possible. If the man is a senstive, caring and spiritual person, he will have the kind of relationship in which his wife will be able to speak freely without making him defensive or resistant. If he is one of those autocratic patriarchal Christians/Messianics who walk more in the Old Covenant spirit than the new she will indeed have a difficult task for he is then a part of a much wider problem, viz. of having a distorted view of true Christian/Messianic Patriarchy.
My simple answer, which hopefully embraces all situations, is this: that the wife should use gentleness, meekness, and genuine, unpretentious love to reach him. There may, of course, be exceptions to every rule - if it is a life-and-death situation involving a child and his error is risking that child's life, then a sharp rebuke may be necessary to bring him to his senses: in this situation she would need to break a lesser law (respecting her husband's authority) in order to obey a higher one (saving the life or health of a child), but hopefully this will be rare. Most domestic disagreements in Christian/Messianic polygamous families are not of this kind.
Where there are many wives in a family, as is the case in polygyny, then it may well be that one wife is better skilled at handling an erroneous husband than another. In which case the wives may wish to council together (so long as it does not degenerate into a 'plot' to undermine him generally) and agree who might be the best to approach him. Again, the Spirit is paramount. Approaching him as a 'gang' might not be such a good idea either - once again, circumstances and the direction of the Spirit are all-important.