From what I know of these things, when you are writing about an already established universe you are given a framework that you are meant to keep within. You might also be given an idea of where the current storylines are headed, so any characters you introduced and stories you start will fit with that. Then either you are given a specific storyline they want covered or you do a synopsis of your ideas for a story and run it by the main team to get their approval, after which you can go with it.
From my experience with the Warcraft universe and those novels, yes, that seems to be exactly how it works. None of the current Warcraft novels are written by game writers, and have not been for some years. But they do tend to work closely with the game devs and are basically supplemental game material; a
lot of WoW expansion and NPC character backstory has been told in the novels. They essentially have a pattern now where they will release a novel in the lead up to an expansion that, erm...
expands, on the story. This has led to similar complaints to those that we've seen on these forums, that of using these outside materials to tell the story instead of showing it in the game.
So either Rucka had a very tight brief that he had to keep to or Bioware vetoed his preferred storyline in favour of the one given or he simply didn't have much imagination when it came to developing the Magekiller idea. I repeat, there was nothing wrong with keeping to the same timeline as Inquisition and having a story running in parallel to it; the problem would seem to be having the main characters for the most part cover exactly the same areas that we did in game when there was so much scope for introducing new ones and fleshing out plotlines that had only been given a small amount of detail at the war table. I can see why they didn't want to put too much in from Tevinter (just enough to wet our appetites) because it has been heavily implied that is where we are headed next game but fleshing out storylines in the south in areas we have never visited or are likely to visit in the near future would have been ideal, so it seems odd this didn't happen.
I agree on your reasoning of not showing too much of Tevinter. However, I really would have preferred that our two main characters had nothing to do with the Inquisition whatsoever. Or if they absolutely
had to be involved, then yes, show them doing something NEW that I didn't already do in the game. And you know, having them return to the Hissing Wastes and showing that the Venatori were back didn't show me that the world is "alive" or "dynamic," or anything along those lines. Instead it made me feel like my Inquisitor's efforts were for nought. It
greatly irritated me.
Mind you I wonder just how carefully the main team follow these writing projects and check them for accuracy or how invested in the game world these authors are. In the Last Flight the writer had Garahel joke "I did once seduce a Qunari", which would be funny except for the fact that the Qunari didn't arrive in Thedas until 100 years after the 4th Blight. So clearly the author was not sufficiently immersed in the game world to know this comment was entirely out of place and the editor didn't read it carefully enough to spot the anomaly (or didn't know either).
I might be inclined to say that this is the problem with having non-game writers doing these ancillary materials, but there have been inconsistencies in Bioware's own works as well, so...