Rereading TTT, the final scene between Cnaiur and Moenghus, and this struck me...
"Suddenly Cnaiur could feel it: the miles of earth heaped above them, the clawing inversion of ground. He had come too far. He had crawled too deep.
The sword dropped from the stranger's senseless fingers, rang like something pathetic across the floor. His face broke, like a thing wrapped about twitching vermin. The sobs whispered across the pitted stone."
Was Cnaiur a skin-spy?
Also, when Cnaiur sees Serwe after Moe dies, I just assumed that he was seeing one of the 2 skin-spies that came down with him, but I suppose there are other interpretations: 1) it's just a hallucination, 2) it's one of the skin-spies that had been captured by Moe, 3) Serwe or something in Serwe's guise is appearing from the Outside.
Also also, I wonder if Cnaiur's feeling of going too deep is smaller version of what the nonmen felt. Of what Cleric aludes to, I think at the end of TJE in his conversation with the Wight.
EDIT: Isn't there something in Dune about the significance of having a face-dancer believe that they were in fact the person that they replaced? Musing about why it would be neat to have a skin-spy Cnaiur who really believes he is Cnaiur