Some comments and thoughts--
4) This is going to require a wizard, I think. There are realm spells which create or possibly could be modified for entry/exit into the Shadow World. The most obvious is "Shadow Gate", but that is a 9th level realm spell. Spells like "Arcane Transport" could have potential (though it would require some research, I suppose). Alternatively, there are some considerably more dangerous ways to do this, I suppose.
5) With the oddity in this was the fallen Paladins, but then I realized that Rournil Paladins are the only paladins likely to traverse the Shadow World, and probably were directed against the Gray Duke. They are a reminder of the price of failure, I suppose.
6) The Wardens are probably a good start for this.
9) I am not sure it really matters, per se. If he didn't take the book, he will still need to be put down.
11) He has definitely infiltrated Elinie & possibly Diemed (though I doubt he had a need to keep them after the theft). He is the Lord of Vampires though, so it is very plausible that his progeny are in many major cities (think level 5+). Less likely in smaller places, I think, since missing people would be more likely to be noticed, AND people that work at night would be noticed and gossiped about. ONe of the tragic flaws regarding Vampire: Dark Ages -- in midieval times, people don't go out at night...they just didn't.