How can one atone for the sin of establishing a romantic relationship with a woman before her marriage, after she was in the process of divorce, and is marrying her the optimal solution for God's pleasure and repentance?

Repentance is obligatory for an emotional relationship if it leads to something forbidden. Helping a woman with divorce procedures is permissible unless it causes her corruption against her husband. The family's opposition to a marriage does not legally prevent it, but if the parents object and insist, their obedience takes precedence, unless there is a fear of falling into sin, in which case chastity takes precedence.