Is merely praying for the religious teacher whom I slandered and threatened to expose his private conversations sufficient, given my inability to communicate with him?

It is obligatory to advise the teacher and point out his mistake without defamation. If you speak ill of him or publicize his sin, then repent to Allah. If he persists in his actions, continue advising him in an appropriate manner. If he repents, then speak well of him and supplicate for him.