Is it permissible to pray Salatul-Istikhara regarding divorce, even if the reason for it is the interference of the husband's family and their insistence upon it?

It is not the husband's right to force his wife to relinquish any of her rights. She is entitled to the rights of a divorced woman if he divorces her. It is permissible for the wife to forgo a portion of her Mahr or to retract her condition and agree to live with his family. The best approach is to choose what is most beneficial and to perform Istikhara. However, once a divorce has occurred, there is no Istikhara concerning it.