Yes it is true. John Dower in Embracing Defeat recounts one tale of a woman listening to the broadcast with her whole village, and she couldn't really understand what the Emperor was saying. But, iirc, he also writes that there would generally be at least one person in each village who'd been educated and was able to relay what was being said. In the above story it was someone who had recently come from Tokyo who said "this means that Japan has lost". You can read an excerpt including this story here.

I can wholly recommend Embracing Defeat if you're interested in Japan's reaction to the surrender and subsequent occupation. It is, in my mind, an example of the kind of standard all historical works ought to aspire too. Rather than just give you a narrative, it does as much as it can (imo as much as is humanly possible) to tell you what people experienced on all levels of Japanese society and to a lesser extend within the American occupiers.

I can't answer your other questions, so I'll leave that to those who can.

How true is this, not only for the reign of Hirohito, but other emperors? Did they ever address commoners, or did they completely cloister themselves off from the masses?

Prior to the Meiji Restoration in 1868, they were almost completely unseen by the general population. Obviously technology played a part in that, but even with that in mind it seems bizarre. Apparently, upon being told that the Emperor had been restored to his rightful position, many in Japan didn't know that they even had an Emperor.