Is a husband considered sinful if he divorces his non-Sunni wife through a Sayyid affiliated with her madhhab, but the wife did not attend court to ratify the divorce and refused to attend after being notified three times, knowing that he gave her all her rights?

What was mentioned does not necessitate that you are a sinner, and the basic principle is the acquittal of responsibility. If you uttered the divorce of your wife or wrote the divorce with the intention of divorcing her, then the divorce has occurred. Its documentation in court is merely a confirmation of its occurrence.