Does divorce take place by writing, and will Hajj expiate the sin of my staying with him while I am divorced from him? Is it lawful for him to compel me to return? Do I have a way out in Allah's law, knowing that I have relinquished all my marital rights and absolved him of all his duties?

Divorce by writing takes effect if the husband intended it. The wife has the right to seek divorce if the husband mistreats her by beating, insulting, or reviling her without justification. She can prove the harm to the court with witnesses to obtain a divorce, and the divorce issued by the court is considered an irrevocable divorce. As for the document written by the husband for your official, final divorce for a specific purpose, it is considered a divorce in courts, but it is not considered a divorce between you and Allah. The husband has no right to take anything from his wife's money or her father's pension except with her consent.