Why does Mumbi offer herself to Karanja when he tells her that Gikonyo will soon be released from detention center?

Mumbi explains the story of her infidelity to Mugo, but does not tell it to Gikonyo. Why? What stops her from telling the story, even though it may excuse her actions?

Explain the transformation that occurs within Gikonyo at the end of the novel. Is he truly poised to understand and forgive Mumbi, or does he simply intend to forget the past?