Ruach Hakodesh is the opposite of psychic. A psychic tries not to know anything about the topic he is divining. A Baal Ruach Hakodesh asks many questions. Also, a psychic has a feeling or bias, which is assumed to be accurate when he has no prior bias. Ruach Hakodesh is unmistakable.
And yes, there is a very big difference between Nevua and Ruach Hakodesh. The main one is that only a Navi carries a message from Hashem to people, and can say, ?? ??? ???. A Baal Ruach Hakodesh, even when he has clear knowledge of something is never saying, I have a message to you from Hashem.
As for ZD’s Taana, it is unethical to go against Hashem’s wish and divulge that which wasn’t meant to be revealed. I know of Tzaddikim who tried getting others to stop someone from going somewhere, while they couldn’t tell anything to the person himself.