Did the husband err against his wife when he married his divorced wife while she was giving birth, and then she (the new wife) discovered his betrayal and was patient, and after his divorced wife confessed that she had performed black magic to cause their separation, the husband's stance was passive, or did the wife err by being patient with him and enduring him even to the point of "his diminished masculinity"?

There is no objection to your husband marrying his former divorcee, and this is not considered a betrayal of you. Allah has permitted a man to marry up to four women, provided there is justice and he is able to fulfill their rights. You must be patient with jealousy, and it is not permissible for you to mistreat your husband or taunt him. You are allowed to demand your rights if he becomes unjust or oppressive. It is not permissible to accuse his first wife of witchcraft without proof. We advise you to perform Ruqyah (Islamic exorcism/healing), and it is not permissible to seek divorce except in specific circumstances.