Which is more absolving of responsibility: for the wife to remain under the authority of her husband, with her apparent verbal consent to her husband's inclination toward her co-wife, or for her to be divorced?

There is no blame on the husband if one of his wives agrees to waive some of her rights. Indeed, Malik narrated in Al-Muwatta' that Rafi' ibn Khadij agreed with one of his wives that she would waive some of her rights, so he kept her. Similarly, Sawdah bint Zam'ah gifted her day to Aisha. It is not advisable to divorce a wife if she is content with living with him, unless it causes her harm or temptation in her religion. And among the noble manners is to fulfill all her rights, even if she waives them.