Is the marriage contract still valid if it becomes apparent that the wife was not a virgin at the time of the contract, despite that being stated therein? And is the husband considered a “dayyuth” if he forgives her in order to preserve the family? And what is the Sharia ruling on his continued marriage to her, and how can she repent?

The marriage contract is not invalidated by the wife not being a virgin. If the woman repents from immorality, there is no harm in keeping her, and this is not cuckoldry (dayatha), for cuckoldry is condoning immorality among one's family. Repentance is by abandoning the sin, regretting it, and resolving not to return to it. If the sincerity of her repentance becomes apparent, then keep her and be diligent in protecting her and keeping her away from temptations, and do not be absent from her for long periods. However, if doubt arises and her repentance is not sincere, then divorce her, for divorce in this case may be recommended or obligatory, due to the detriment to religion and corruption of the marital bed it entails.