I would consider canon at least the stories from Andrew Greenberg, Bill Bridges and, maybe, Daniel Greenberg.
Also the book is edited by Bill Bridges, so the other stories could be cannon as well, and it is a published book by Hollistic. Obviously each is up to every GM to decide what is canon in his Fading Suns. I still need to read the Town's Criers Guild news, maybe there is something there tied to the stories in The Sinful Stars.
About the Decados, and without spoiling the story, looks like Hiram was just playing a bit with that option, it doesn't seem he make the pact himself but uses pawns for that, which is closer to my view of the Decados
