Is there such a thing in Sharia as "Nikah Jihad" (marriage jihad)? If so, what are its conditions? What is the ruling on those who speak about it without verification? And does this fall under slandering chaste women?

There is nothing in Islamic law called "Nikah Jihad" (marriage jihad). Marriage has its own conditions and pillars. The Sharia has safeguarded honor and made slandering it unjustly one of the greatest sins. It has also ruled that those who accuse chaste women of indecency should be flogged, have their testimony rejected, and be deemed immoral. So, if someone accuses a chaste person of immorality and fails to produce four witnesses, the accused has the right to bring the matter before the ruler to have the punishment for slander (hadd al-qadhf) inflicted upon the slanderer.

It is incumbent upon a Muslim to guard their tongue and ears from delving into the honor of believers. Those who lead campaigns of defamation and spread lies are criminals. Allah, the Almighty, says: "Indeed, those who like that immorality should be spread among those who have believed will have a painful punishment in this world and the Hereafter. And Allah knows, and you do not know." (Quran 24:19). And in the Hadith: "Any man who spreads a word about a Muslim man, while he is innocent of it, thereby slandering him in this world, it is a right upon Allah that He melt him on the Day of Resurrection in the Fire until he brings forth the fulfillment of what he said." Similarly: "Whoever mentions a person with something that is not in him, so as to disparage him with it, Allah will imprison him in the Fire of Hell until he brings forth the fulfillment of what he said about him."