This is a very fair concern, Diedne are interesting because of the mystery and it’s hard to preserve that when you examine them in depth
Here’s a thought, what about a book about the Order right before the Schism War? There’s no way they could stretch Diedne to fill an entire book anyway, the HoH books cover four apiece, so if they did explore Diedne it would have to be included with something else. The presentation of House Diedne could be largely mundane, or at least as mundane as a mystery cult can be, and it would leave open the question of what Diedne did preceding the Schism War (if in fact they did anything). Even if they weren’t corrupted by an infernal or faerie power Diedne managed to give the Order a run for their money which is impressive for a single House. If they were influenced, well that’s something that can be hinted at or discussed briefly but they don’t need to go in to details thus preserving much of the mystery while still providing a glimpse. Done well this could actually reinforce the mystery rather than expel it.
The book could give a short overview of the Houses and Tribunals at the time, as well as some key points about Europe at the time. It could look at the political situation in the House in more depth and perhaps provide some key locations of the upcoming Schism War if the troupe wants to run a saga through it.