Is disparaging shaykhs and preachers online considered backbiting and slander? And is the publisher of Islamic video clips sinful if it leads to discord or criticism of them?

The flesh of scholars is poisoned. Backbiting is to mention a Muslim with what he dislikes. People speaking about scholars with what they dislike is forbidden backbiting. Backbiting is forbidden in the case of ordinary Muslims, and even more so in the case of callers to Islam and reformers. It is obligatory to advise these critics, and to defend the honor of scholars. Whoever publishes da'wah clips to benefit people will be rewarded, and he is not harmed by some people dealing with them in a forbidden manner. Everyone who publishes something good will be rewarded for it.