Will the perpetrator of the grave sin of fornication, who regretted and repented sincerely to Allah, be forgiven on the Day of Judgment, even if the Hadd (prescribed punishment) for fornication was not carried out on him in this world? And is repentance alone sufficient to expiate this sin?

Executing the prescribed punishment for a sin atones for the sin and its guilt. Likewise, sincere repentance achieves the same. Allah Almighty says: ﴿Except for those who repent, believe and do righteous deeds. For them Allah will change their evil deeds into good ones. And Allah is Oft-Forgiving, Most Merciful.﴾ (Qur’an 25:70). So, whoever is sincere in repentance and frequently seeks forgiveness is not obligated to confess in order for the prescribed punishment to be carried out; rather, repentance is sufficient. It is recommended for one who has fallen into disobedience to repent to Allah, conceal their sin, and not mention it to anyone. The Prophet, peace and blessings be upon him, said: "Avoid these abominable acts which Allah has forbidden. Whoever commits any of them, let him conceal himself with Allah's concealment and repent to Allah. For whoever reveals his misdeed to us, we shall apply the Book of Allah the Almighty to him."