Is what the questioner and his fiancée did—in terms of prayer, fasting, Zakat, and supplication for concealment, marriage, and sincere repentance before and after marriage—considered repentance for the sin of fornication, and is their marriage valid?

Performing obligations and supplicating are not sufficient for repentance; rather, one must hasten to a repentance that fulfills its conditions.

Scholars have differed on the ruling of a fornicator marrying the one with whom he fornicated before repentance, and some of them have held this marriage to be valid.

One must be cautious about being lenient in the relationship between a man and a non-mahram woman, due to the temptation and corruption inherent in that.