Is there a consensus among Muslim religious scholars regarding the permissibility of reviling an openly defiant sinner, and does this contradict noble morals and the stature of the Messenger, may Allah’s prayers and peace be upon him, and does one who objects to this commit an act of disbelief?

The fatwa (definition) did not mention reviling the one who openly commits sins. We have previously explained that a gentle and kind approach in da'wah (inviting to Islam) and guidance is more effective and beneficial. This was the method of the Prophet, peace and blessings be upon him, in enjoining good and forbidding evil, as occurred in his stance with the young man who asked him for permission to commit zina (unlawful sexual intercourse), and with the one who prayed incorrectly.