What is the religious ruling on marrying a girl who became pregnant by me after an illicit relationship and then had a miscarriage? And is the عرفي (customary) marriage contract that was not formalized considered a valid contract before her father swore an oath of divorce? And how can one atone for this sin?

The questioner must immediately repent of fornication. His marriage to the girl is not a condition for repentance. An informal contract is not a legitimate contract without fulfilling the conditions of validity, such as the guardian's permission and the presence of witnesses. Aborting the fetus before forty days is an issue debated among jurists. The father's oath of divorce has the ruling of a conditional divorce, and the divorce takes effect if the condition is met, according to the majority of scholars. The girl's father does not have the right to prevent her from marrying a suitable man without a legitimate reason. If the guardian withholds consent (adhl), she may refer the matter to the judge. If the marriage will cause the father to break his oath and result in harm to the parents, it is better for the girl to refrain from marrying him.