I think the Hero's inclusion in Inquisition, even in letter form, confirms that the Hero will return. Jaws of Hakkon also implies that the Inquisitor is not opposed to the concept of tracking down legendary figures.
I doubt it's that easy. The other magisters will likely spread to a new blighted host like Cory did. However, a codex in the Deep Roads in Inquisition confirmed that one of the sentient darkspawn ate another one. This was probably Corypheus. So presumably a magister can be killed easily by another magister. I don't think the Architect was a magister though. Unless he simply forgot everything. Plus, he didn't transfer to a new blighted host. At least, not that we know of. I killed the Architect.
Well, we killed him, but that doesn't mean he didn't transfer after death. Also, he said he was born like that, but he could have meant after he was cast down from the golden city. And he doesn't look like other darkspawn, he looks a lot like corypheus.
Just because he didn't tell the hero he just met he was one of the original darkspawn magisters (because why would he?) doesn't mean he isn't one. And it would be pretty shoddy exposition if he told a grey warden he could take over other darkspawn. He could make a return easily enough, just in one case he and the warden would have parted on good terms, and on the other, he'd be very angry - would make an awesome scene.
Why do people still assume the villain is going to tell you absolute truth about their history and all their weaknesses when it doesn't help them to do so?
I think he's a magister. And he knows a lot about the calling. Potential for great writing is there either way.