What is the ruling on a husband kneeling to kiss his wife's hand while putting on a ring, and what is the Islamic advice for those who backbite and speak ill of this action?

Kneeling was a custom among Arabs when experiencing fear, exalting an matter, or showing respect and submission. It has been mentioned in multiple hadiths indicating its permissibility and that it is not originally forbidden, unless it involves imitation of disbelievers or another شرعي (Shar'i) prohibition. However, the kneeling mentioned in the question (for the husband before his wife) is not a custom of Muslims and is likely a custom of the polytheists, and it is not befitting for a man. As for kissing the hand, it is originally permissible for a scholar, a ruler, an elder, and parents, and for the wife if custom does not prevent it, though it is considered unseemly to do so in front of people. It should also be noted that wedding parties where unrelated men and women mix are not permissible, and it is obligatory to denounce evil to its perpetrator rather than speaking about it in his absence.