I'm not asking you to accept anything I say outside of the plain facts, which come with their own "credentials" and I think are subject to some discussion without the permissions of an authority figure. I don't know enough about these companies decision making processes (where the actual law is often of only passing relevance) to have an educated opinion on the basic question of why the outer planes as defined by DnD haven't been published by other companies. I know that for a time Mayfair was being sued for publishing DnD-ish products. And so I was posing a theory, that I think has some substance, that the answer may in part involve legal issues/concerns (which clearly NG has dealt with in some way).
NG's City of Brass could be an exception because of it's history. I haven't seen anything to rule out that NG laid some groundwork with WotC before/during it's deal with RJK. Otherwise it's somewhat interesting that the original author (RJK) yanks his work from them and they somehow still decide to contract with new authors and create the product. Other than the legal groundwork being lain, the only other explanation I can think of is that there was a substantial amount of maps/artwork or whatever (though I wonder if such things ever constitute a major fraction of the cost of production).
And again, there is AFAIK an absence of any other product from the milieu in question being published by the company with the "Old School feel". If legal issues weren't involved (and I haven't seen anything where you have directly said that anyway) then why else would it be? Why would a company with "Old School" inclinations pass up the chance to create a Nine Hells boxed set? Or a Gehenna module? Especially when they can so quickly turn around and create a City of Brass with what appears to be short notice (and the probability of facing a competing product from RJK).
I don't think the LotR/SoS is a good analogy. The analogy would be as if some author wrote a story about a place called "Camelot", but the castle was made of cheese and inhabited by horse-headed humanoids. And then another author came along and wrote a different story where a castle named Camelot were made of cheese and inhabited by horse-headed humanoids.
I haven't seen NGs CoB, but I'm assuming that it's a city situated on something like an elemental plane of fire and inhabited by efreeti/ifriti/whatever that resemble the DnD monsters of that name far more than the ifrit of legend. This bears virtual no resemblance to the CoB of the Arabian Nights. No more of a resemblance than the above "Camelot" does to the legendary one.
Now the details of what the actual law is, and what agreements existed between the various parties I have little sense of. Nor do I have an opinion on anyone's character involved in this. But I do have a feeling, as I've said, that if I published a comic book named "Thor" where the main character had a biography that strongly resembled that created by Marvel, that I would be sued. And protests about the existence of Thor as a Norse mythological figure would not be guarranteed save me.
You do realize many people have written stories about Camelot, King Author, etc... even using the same names of characters and locations, but did not copyright each other.
Copyright infringement only occurs when you use the work of someone else. IE you copy them word for word. You can use the common ideas, but as long as you wrote the story yourself and didn't copy anything from the other authors work, you did not commit copyright violation.
Now world settings, like Dragonlance, are Trademarked. So you could not write a story using anything that recognizably defines the Dragonlance setting, but you could write a story of a bipedal race developed from dragons. However there better be a lot of originality to it, or a infringement lawsuit could occur.
Copyright law is not this strangling straightjacket that so many seem to think it is. However, if you have read a lot of books, you have already seen a lot of similarity. Heck, the romance genre is written to follow a general formula, but they aren't copyright infringements.
So just because Necromancer and Rob Kuntz both did a City of Brass write up there is no copyright infringement unless one directly copied the work of the other, word for word, or Art wise. Even for Art you have to copy the work to a degree where you would be able to pass one off as the other. Or prove the drawings are the same, just colored differently. Otherwise its still an individual interpretation of the same subject matter. IE the same as two artists painting pictures of the same scene, object, or subject. IE they both painted the same thing, but did so with their own work, skill, and interpretation.
So don't worry about it, if you are.