How can the execution of the hadd punishment for adultery on the married adulterer, by stoning to death, be reconciled with the mercy and justice of Allah, while the hadd punishment might not be carried out on another adulterer, thus allowing him an opportunity for repentance?

A Muslim must believe that Allah Almighty is just and wrongs no one. Justice means putting everything in its proper place, not treating all people equally in every respect. Therefore, it is not an injustice that one of two fornicators had the prescribed punishment (hadd) applied to him while the other did not, because variations in divine decrees are not injustice, and the prescribed punishments (hudud) are expiations for sins. A creature cannot encompass Allah's wisdom in His decree; rather, one must submit and yield to the belief that Allah is Wise and Just in all His actions.