Imagine you are Nene. Write the story of "Marriage is a Private Affair" from the point of view of Nene.
In writing the story from Nene's point of view, I would stress the modern conception of marriage as being essential. While Okeke reflects the past vision of marriage and of being in the world, Nene is the future. She is an educated woman, and embodies the upward mobility that the traditional structures of caste and tribe deny. She lives in Lagos, an urban center, and is able to recognize that women like her have benefits that have not been present in the past. Her point of view would stress this progressive tendency, overcoming the stubborn and obstinate condition of Okeke.
I would focus on this when she encounters challenges in her marriage to Nnaemeka. For example, when she receives their wedding picture with her head cut out of the picture, it might be a moment where she has to remind herself of her internal resolve, a resolve that has moved her to a particular point. It is this resolve that continues to drive her against those who say she cannot do something or pursue what she wishes. Focusing on this would be help to enhance her narrative and explain why she ends up writing the final letter to Okeke, taking the stance that she does.