Still around and checking in! Glad you enjoyed the book! For your questions:
1.) I think the primary links between my books are mostly themes and tiny details. I like themes that are darker for sure, characters that are pushed toward moral decisions that, in the hands of more clear-cut 'heroes' and 'villains', might take a different turn, and I enjoy having large stakes on the table. In
Bloodwalk it was a town (though the implications of taking the town could have threatened the region). In
The Shield of Weeping Ghosts, it was a city and a hellish power, possibly sparking a losing war for the wychlaren against their enemies the durthan. In
Restless Shore, it took a much more Lovecraftian turn in that the villains' conquest was incidental to their enthralling gospel. And in
Circle of Skulls, if Asmodeus had been successful (as he was the true off-camera villain/puppeteer), the whole of the Realms could have been at stake (see answer to question #2, heh). So my stakes got bigger and bigger. And details-wise, there's a book that appears in
Shield of Weeping Ghosts that bears a striking resemblance to one of Morgynn's spellbooks in
Bloodwalk;
Restless Shore didn't have any easter eggs actually, 'twas its own beast, heh; and in
Circle of Skulls, there was Bastun, giving my favorite character a place and a future in the Realms (and a whole 100 years of untold story, gah!).
2.) Though Wizards doesn't necessarily call us up and tell an author their reasoning for not offering another novel (plus all my contacts within Wizards have been fired, left the company, or no longer have the access they once had to internal decisions) I suspect it was for obvious reasons why Jinn and/or Bastun did not get a continuation. Many of the themes in the 4th edition Realms were more 'high-adventure' (and definitely not "high-fantasy" as it was described in the original style guide), much more related to their gaming audience (obvious for novels based on a game). And my novels were dark (and getting darker at the same rate as my stakes got higher), less adventurous, more horror-related, much less connected to the game rules (wasn't a fan of the 4th edition rules, especially magic, really struggled with magic writing
Circle of Skulls). And as many have said, my books don't have that Realms "feel", which is odd to me when people think of that as a detrimental point, since the Forgotten Realms is an entire world. Look at the fiction section in any bookstore, what's the "feel" comparison between say "Fight Club" and "The Great Gatsby"? They take place on the same planet, yet they have two different styles and feels, themes and characters, etc. Seems to me that horror is a part of any world, especially one with magic. Magic by itself is not horror, but in the spaces between spells, where things are forgotten and that energy is left to intermingle and make all kinds of interesting beasties...anyway, for another point: stakes. I read one of the old rules for Realms novels once, a guideline that goes "Don't blow up the moon!" And ever since I read that, well, I had an irrational urge to blow up the moon! lol
So, really, who knows what the actual reasons are, but I don't doubt that a few of those may have played a part (for the record my sales appear comparable to other books in the same series). Moving forward though, my current novel project definitely does not have a Realms feel (lower fantasy and much higher horror content).

And unless there is another Waterdeep book that I'm not aware of, I thought there were only six in the series:
Blackstaff Tower, Mistshore, Downshadow, City of the Dead, The God Catcher, and Circle of Skulls, right?
Thanks for the questions (keep 'em coming if ya' got 'em) and I hope my answers don't disappoint!
--James