I listened to the audiobook so I don't have page numbers, but there are several descriptions of servants being badly beaten. The woman who is married to the abuser is beaten and her arm broken. She expresses fear that even if she leaves he'll never let her go. The woman is later beaten to death.
Yes, several different persons, but in each case the violence and the mental illness are separate things. They are mentally ill, and they are forced to do violence. The mental illness is not the reason for it.
By the strictest definition, yes, since one of the characters can and does change their true physical appearance using magic to fit how they're feeling at the time (don't want to spoil more)
There is a relationship between a man and his brother's widow, which is not incestuous by the standards of our culture but is described as incestuous by the standards of their fictional religion.