<Now the question I hate to ask, as aspects of it have been covered before, but I still don't know the answer to: who the hell is/was buried in Sev's tomb, the one he played in as a boy? Who built it? When? Why? Why were there five coffins in it, two open and empty on the floor, three intact on the shelf?>
mantis has addressed this previously and perhaps he'll reprise it or tell you where you can find it in the archives.
My own theory on the two empty coffins is that they've been rifled by Hildegrin the Badger. He tells us in CLAW that he's a done a favor or three for the Cumaean and since graverobbing is his specialty he seems a likely candidate. As for where the bodies wind up, let me recall for you the scene in URTH where Severian at the Chowder Pot is rescued by a number of people from Zama: "Certainly more guests came to our aid when Zama broke down our door than there was any reason to expect, and I would like to think that one or even several of them were myself. Indeed it sometimes seems to me that I caught a glimpse of my own face in the candlelight that night." As for how these "alternate" Sev's wind up back in the time of Typhon, let alone revivified, I'll leave that a mystery for now, but a clue can be found in "Ceryx."
Robert Borski