Do the Hierogrammates *care* about the Megatherians

No, I don't think anyone did. But we may wonder whether this represents actual concealment within the world of BOTNS, or whether it's just Wolfe's tendency to make things less clear than they might be (and then be surprised when people don't get it). We know, now, that Dr Talos's play was based on Canog's book, which was based on Severian's own testimony. So he, at least, was not trying to conceal anything. There's also another passage in Urth that bears on this. Apheta says that the people of Urth have a choice between slow death, and death followed by rebirth. Severian says that he has known this since childhood. Gunnie says 'We didn't', but then adds 'In religion - but no one took any notice of that' (or words to that effect). So I think the destructive effect of the New Sun was known, in the sense that it was not deliberately concealed, but, as with many things, people at large took no notice of it. (One might also mention the Conciliator's title of 'Black Sun', and Dorcas's saying she has read some terrible things about him.)