Should a young man marry the girl who led him to fornication after his family rejected her for being divorced, or should he marry someone his family approves of, and how can his sincere repentance be after committing this sin?

Obeying one's parents in marriage is paramount. The best course of action is to marry someone whom the family approves of, provided she is of good character and religious devotion. You must repent for the sin committed with the woman, increase your good deeds, sever all ties with that woman and with women in general, and return to your country if you have no valid reason to remain abroad.