Well I have read the amicus curiae for the Klingon language, and while the arguments are interesting and entertaining, I am left with the feeling that this is another case of consumers of an intellectual product deciding that they are going to expropriate it for their enjoyment, just as happened with Axanar.
Klingon was not put out into the world as Esperanto, an offering without conditions intended to ease communication between different language groups. Klingon was put out into the world as an artifact of an entertainment production. Its dictionary was published as an entertainment artifact.
The AC authors argue that none of that matters because now, people have expropriated that property and are engaging in activities which, devoid of context, could not be in the reach of copyright. The AC authors argue that new vocabulary was not part of the original films and therefore not part of the studio's scope of copyright, even though the new vocabulary needed to conform with the existing canon of the dictionary and prior Klingon uses.
When you look at it, you see Axanar mentality all over again. That is, the assertion that anyone proclaiming themselves the 'true holders of true meaning' of the IP are not subject to copyright; and that creating stories which derive from the entire canon, but supposedly are primarily adding new material, are also not subject to copyright because the IP holder hasn't gotten around to that story.
I think what C/P will need to do is get specific that they own everything in the published works they copyrighted and licensed, the dictionary and Orkrand's other books, and any additional grammatical or pronunciation precedents were set in the media works that also lay groundwork for substantial similarity analysis. They need to show that the Axanar content bears substantial similarity to these works, not everything every fan ever invented. They need to keep the question focused on this, not on whether an entire language is copyrightable.
A key part of the WS and AC analyses is the point where they claim that because C/P didn't explicitly say they were laying claim to their copyright works (like the dictionary) specifically, that C/P *must* be claiming everything about Klingon. This is easily remedied by C/P stating a clarification that they lay claim to the content of the works they had done for hire or licensed.
An interesting side point made by someone on the HR site's coverage is that if you don't allow for constructed languages in entertainment to be protected, there won't be any, because the studios won't pay for it.
I have no doubt there are examples of IP being expropriated by interest groups throughout recent history. Sometimes it may be successful. C/P need to stay away from being sucked into this broader question, which as a defense strategy is a way to entangle the IP theft case in a sideshow.