Jeff Wilson- That last related tale puzzles me. The very large, very pale "pandours" suggest armed and armored siblings of Idas.
Perhaps the pandours and undines are related. I take these pandours as being the
"cold, pale warriors of Erebus" that are mentioned. Somewhere in the text there is a mention of the reconcilliation of Erebus and Abaia and this scene combining pale warriors with undines and a deeper underwater presence might illustrate the collaboration or conjoining.
In The Tale of the Student and His son, we have the Naviscaput who raped Night, almost surely a reference to Nyx the primordial Greek goddess of Night. Severian overtly associates the Naviscaput with Abaia, but the Greek legend has Nyx mating with her brother, Erebus. Perhaps more evidence that Erebus and Abaia are basically aspects (portions?) of the same being.
In Greek mythology, among the offspring of Erebus and Nyx is Charon, the Boatman
of the dead. Many find an allusion to Charon in Dorcas' husband, the tiny old man so "bowed and shrunken by age" who describes Father Inire with, "just a little man he is, with a wry neck and bow legs".
(can't pass up the chance to mention Rudesind here also advocate of the dead).
I see this character as the possible family connection between Severian and the great, alien powers of Urth.