How can we reconcile the fact that the disobedient (non-polytheist) is subject to God's will on the Day of Judgment with the rights of the oppressed? And does God forgive the oppressor even if he has not fulfilled the rights of His servants? And what is the extent of the authenticity of the hadith that refers to this, and can it be relied upon?

There is no contradiction between the two fatwas, because the general principle is that sins related to the rights of people necessitate retribution through the exchange of good deeds and bad deeds. Thus, the wronged person is fully compensated for their right, and retribution is taken from the wrongdoer. As for the specific exception, it is that Allah Almighty may appease the wronged person on the Day of Judgment and compensate them, and forgive the wrongdoer for a wisdom known to Him, glory be to Him. Allah may also overlook the wrongdoer's consequences and compensate the wronged person with what pleases them. It has been stated that the hadith which indicates that Allah appeases the wronged person on the Day of Judgment and overlooks the wrongdoer is weak.