Right... I forgot about the archivist. Those books were such a fucking mess in terms of the 50 characters constantly being referenced(although not as bad as some of the 40k books at times), and the constant recycling of names or using similar names and characters showing up purely for the sake of making an appearance.
I didn't see it as much as a mess as others, instead that a lot of it was understandable as to why characters/plots were inserted.
Whether they paid off or not is subjective to the reader-isn't the point here, only the why, especially when taking into account Abnett was proposing to GW was to"shake the foundations of the setting".
Now after the main HH and Siege novels have been released, seems to me that his idea was about filling the gaps/expanding of a central mythology he created/set up via his Inquisition series, and developing/backfilling it throughout the HH series.
The current theory is Valdor is the King in Yellow, personally don't think it will be him, even after his revelation from seeing the future but could see Valdor assisting the King being either a perpetual; as not sure why Olanius couldn't regenerate when Horus swiped him, Gramaticus who was last seen time hopping, a Primarch-Dorn would be obvious choice for the colour but could be Russ with the King in Yellow being a name corrupted by time like others Abnett used. The extreme tie everything together would be the shard of the Emp being actually reborn in a child, which has been presented in the Dawn of Fire series
Using primarily the theme about the "Cold War" with Inquisition and Cognitae over the 10,000 years upto present 40k in the Eisenhorn/Ravenor/Bequin series , the different aspects of Inquisitorial philosophy Amalalthian, Horusian etc has actually been a longer running dispute over how to fight Chaos and was the basis for the original schism of thought between those who wanted to use Enuncia or Chaos against itself, which was Actae's position, mirrored by Abaddon's "Control, not Controlled".
We know the proto-Inquisition founding members are all around the Imperial Palace at the immediate end of the Siege so the idea of using all means to fight back will be tolerated but as the Scouring winds down-approx 200 years and the Echlesiarchy's rise, this is the likely split from the original members positions with possibly Moriana and Chase both each going separate ways, or Sinderman casting them out as with him being faithful, can see him having the political position to do so whereas knowing Sinderman is over 1k in age when he cops it in the War of the Beast, he recognises the inevitably of the above philosophies so can no longer excise it and which is why by 40k time, these approaches are "acceptable".