Which is safer in terms of clearing oneself of liability: to keep a wife who pretends to like her co-wife, or to divorce her?

All perfect praise be to Allaah, the Lord of the worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad, sallallaahu ‘alayhi wa sallam, is His Slave and Messenger.

There is no harm or blame on the husband if one or both of his wives willingly agreed to give up some of her due rights seeking to remain in wedlock or for any other reason.

Imaam Maalik cited in his book Al-Muwatta’ that Raafi' ibn Khadeej   made an agreement with one of his wives to give up some of her due rights and he kept her as his wife on that condition and he did not feel guilty that she accepted it.

Moreover, Al-Baaji highlights in his explanation of Al-Muwatta’, “If the wife approves of this condition, it is permissible. Sawdah bint Zam‘ah gave up her night [of sleep over] to ‘Aaishah seeking to please the Prophet, sallallaahu ‘alayhi wa sallam, who allocated two nights of sleep over with ‘Aaishah ." 

Hence, we would like to advise you to keep your wife if she agrees to live with you unless this might bring about any sort of harm or Fitnah for her (temptations in her religion). Moreover, it is worth mentioning that if you manage to fulfill her due rights, even if she gives them up, this would be a praiseworthy act of highly refined manners.

Allaah Knows best.