Suggestion for the secret of the inhumi:
Everyone's been focusing on a change in human conduct as the secret that can destroy the inhumi. To me, the most easily accomplishable and plausible weapon that humans could have against the inhumi would be the ingestion of some substance. Since the inhumi drink human blood and appear to derive their personalities from the blood they drink, there may be something that humans can drink or eat that will impact the inhumi negatively. This is pure speculation . . . I have no supporting text to cite.
*This is WHORL, for discussion of Gene Wolfe's Book of the Long Sun.