@Durin I have a few questions if you don't mind.
1. By the Ancient One's presence being "too distinct", do you mean that no one would believe he'd ever work with Chaos/Tzeentch, even temporarily in order to rescue Isha? It sounds a bit like you thought I was suggesting he and the others would pretend to be Tzeentch worshippers, which wasn't quite what I meant.
1.a) As an alternative to pretending to have temporarily worked with Tzeentch out of necessity to rescue Isha (even if they still absolutely hated Tzeentch the whole time), how about presenting them as unwitting pawns of Tzeentch who he manipulated without them knowing? Would that be believable? And how effective would that be in diverting attention from Avernus/the Trust?
2. So, we know that this will increase suspicion of the Trust by "some" or "a lot". More specifically though, what's our best guess as to whether or not we'll be invaded by Abaddon as a result?
2.a) If yes, how soon do we expect an invasion after Ridcully's identity is discovered?
2.b) If an estimate isn't possible, do we have any idea of whether we would be a priority target, or whether we would just be added to the bottom of a lengthy "things to do" list?
3. You've mentioned that the Eldar would provide free military assistance against a Nurglite incursion on Avernus. Would they do the same if Abaddon decided to attack the Trust after Ridcully's true identity is discovered, since that would also have been caused by rescuing Isha?
4. Regardless of 3, can you say approximately what level of favour the Eldar's free military support would be equal to? E.g. A major favour?
5. If we faced an existential threat like an invasion, would we get any free support from Quartus/Secundus/Callamus?
5.a) If yes, what magnitude of favour would this be equal to?
5.b) If no, could we go into some sort of favour "debt" to get support from them anyway?
6. Can you pay the cat tax and share some pictures of the kittens please?