Is the father considered to be at fault for abandoning his daughter and refusing to attend her wedding, despite her repentance, and how can he be persuaded to reunite the family?

Your father is wrong in boycotting his daughter after her repentance. You should try to convince him to forgive her and seek the mediation of those he respects. The daughter must strive to please her father. Her father's consent is necessary for her marriage. If he prevents her from marrying a suitable suitor, she can refer her case to the judiciary. You are not permitted to attend her wedding without his permission. It is better that he permits it and expiates his oath. If he orders you to boycott her, there is no obedience to him in that. It is better to try to reconcile with him and convince him to forgive her.