Does not going back to the matchmaker prevent Allah’s forgiveness and pardon for the transgressions that occurred between them? And do completing the engagement and marriage expiate these sins?

The engaged couple must repent to Allah Almighty for any transgressions. Repentance involves abandoning the sin, regretting it, and resolving not to return to it. This is not achieved by going back to the engagement and completing the marriage. There is no harm in returning to the engagement and completing the marriage with her, while emphasizing concealment and not openly publicizing the sin.