Will the one who was a cause for a disobedient person's repentance and abandonment of sins for the sake of pleasing his Lord be rewarded, despite the continuous calamities befalling him and his fiancée, which led to the family's refusal to complete their marriage?

"The one who points to good is like the one who does it." And whoever is a cause for someone's guidance will receive a great reward. However, one must be cautious of relationships between non-mahram men and women, even if the intention is da'wah (inviting to Islam), because Satan corrupts such relationships and opens doors to temptation.

If the man repents and his repentance is sincere, then there is no objection to your marrying him. And your family is not permitted to prevent you from marrying him as long as you are content with him and he is suitable.