What is the ruling on marrying a girl who was helped to obtain a khulʿ from her unjust husband? And is that considered takhbib, especially given that she was seeking khulʿ before the questioner knew her?

You have done well by feeling remorse and fear of Allah. Make this remorse a repentance that fulfills its conditions. If the woman is harmed, she has the right to seek separation. Initially, efforts should have been made towards reconciliation. What is forbidden is corrupting her against her husband. If you did not do that, then there is no sin upon you, and there is no objection to marrying her if you did not seek to corrupt her against her husband with an explicit or implicit promise. If marriage is not feasible, then sever all ties with her, for it is not permissible in Islamic law for a Muslim to have a relationship with a woman who is a stranger to him.