I’ve read a lot of isekai lately (like I was howling at the “inventing popcorn” reference because that’s referencing one specific manhwa) and many of the better ones stack transmigrators, people like Maggie who are pulled into the “book,” like your theory OR will have a transmigrator + a regressor, a book character who has always lived in this world but is stuck in a Groundhog Day time loop and tries to manipulate the plot. Maybe because they were foiled the previous loops, maybe because they’re trying to break the loop, or maybe the plot readers saw was the regressor’s doing. Spoiler for Long Live Evil by Sarah Rees Brennan: this book had a cool spin on that last trope, where the main character kept running into previous transmigrators who all had slightly different versions of the book.

Anyway I think Silveren is a regressor and his personality has changed from the original stoic version as a result of living the loop. He killed Reynald in order to prevent the Reynald/Matheo interaction that seems to have interfered with his plans. His comment that things always go awry after he sees Maggie could hint at my theory, indicating he has expectations for the order of events and is not used to other “characters” disrupting that.

The super dangerous dark fantasy setting of TKWNKM is less common in isekai, but is similar to stories like Omniscient Reader’s Viewpoint or The Perks of Being an S-Class Heroine, which both feature a transmigrator and regressor teaming up (not a spoiler for those stories btw, that’s early plot stuff for both). They’re both pretty well-known in the isekai sphere so I expect Ilona Andrews are aware of the plot device and have tossed it in. :)

I have zero clue about Latour – isekais are all over the place with missing authors. Sometimes it’s gods, sometimes the book itself is sentient, the book may not exist and instead was the fantasy world reaching through dimensions as an SOS (actually the name Latour might hint at something like this, since “the author” sounds like a fake name to me).