AxelSnaxel asked: “I’m curious how that rule applies to cases where a monk could use psychic powers to reveal unseen realms to dispell wrong views?”
– That is likely to be allowed. However, it could be much more difficult to show other realms (say, niraya) to other people. It is one thing to see it for oneself, but showing it to others is likely to be difficult.
– Of course, the Buddha could do that. In one account, some servants of Visaka used to laugh at statements of existence of niraya (equivalent of the hell) saying that is was a myth. So, one day Visaka took them to the Buddha and explained the situation and asked the Buddha whether he could show how beings are subjected to suffering there. So, the Buddha showed them a scene from niraya, and apparently they got so scared that they started following the Path.
You wrote: “I’ve also heard that at the time of Asoka, when Arahants where sent out to the edges of the kingdom to spread Buddha Dhamma (among those Sri Lanka), the arahants often used various powers to convert the people from spirit worshipping.”
– I had not heard that. If you have a reference, I can comment.
You wrote: “Also when the first desana was to be given in Sri Lanka, the novice accompanying Mahinda used his psychic power to announce it to the whole country with his voice.”
– I do not think that is true.
– However, many thousands were people started coming to listen to the desanas. It must have been impossible to speak to such a large audience in one’s natural voice. They I have heard that Ven. Mahinda used psychic powers to “amplify his voice” so that all those people could hear.