GW Elves are one of their least unique creations. Their entire fantasy range is incredibly derivative (as most high fantasy is) but their elves are amongst the worst offenders. Tall, graceful, flowing robes, magical, lyrical voices, deadly in battle but few in number, a dying race being usurped by the numbers and technology of men. Favoured weapons in battle tend to favour spears and swords with slender leaf-shaped blades, armour and weapons tend to be lighter and more intricately wrought than anything man could achieve. They have great affinity for magic and creatures of magic especially including dragons and eagles, and use 'traditional' weapons such as bows in contrast to the more advanced weapons of other races. Tend to get split into kindreds based upon their alignment. That pretty much describes every single elf in every (post-Tolkien) fantasy work. GW cannot copyright this idea (because it is an idea, not an expression) nor can they copyright anything related to their elves which is inseparable from this general idea of elves.
Given that general idea of elves, there isn't much left about GW elves (aside from specific characters in the backstory) which is unique. They're not quite as badly off as the Dwarves are, but very nearly.
GW were not allowed to produce LOTR elves, because those aspects which would distinctly tie the elves to being LotR elves are characters, which are copyrightable (because they're characters). And after Peter Jackson's LotR movies, making elves in that style would have been directly copying PJ's unique copyrighted expression of elves.
CHS does are not producing 'GW Elven parts or models': they are producing parts which are compatible with GW models, using designs which are based upon the concept of elves in general and upon concepts and images in the public domain.
Alan Merrett said that GW would never trademark Halberd. They could probably never trademark Empire Halberdiers either, because the term is almost purely descriptive and non-unique. They could certainly never copyright those, because they are concepts.('Halberdiers of the Empire of Sigmar', on the other hand, could be trademarked as the Empire of Sigmar is a protected work). They could however trademark something more unique, like 'Necron' or 'Cryptek'. A trademark is quite literally a 'mark' (identifying word or logo) used in trade (to sell goods by allowing your customer to identify with it). So in the deposition, it came out that lots of terms which GW claims are associated with their products are not 'trademarks', because no item was ever sold using those identifiers. Tervigon, for example: GW never used that term in commerce before CHS came out with their kit (being written in a GW book is not sufficient to protect something as a trademark, as the word within the book is not generally used to identify the book to customers; and is disallowed on the basis that I could print a book containing every 8-letter-or-less word and tie up numerous trademarks).