Kumori Japanese for cloudiness/shadow.
Langtry, Arthur Last name is Old English for "tall tree" or "dweller by the long tree"; old established Devon family. First name: Arthurian legend is at least here a clear connection, given his position as Merlin. Etymology: several schools of thought; potentially Welsh for "Bear Man" (arth-gwr), or Latin for "brightest star" (Arturus).
Morgan, Donald Last name is Welsh in origin; actual meaning of name is disputed, with meanings ranging from "m'or" (sea) to "mawr" (big) and "cant" (100, cycle) to "can" (bright). Could also potentially (and more appropriately) be related to the Irish Murchadh ("sea warrior"). Donald: Gaelic; either an Irish Gaelic form of the Hebrew name Daniel, which means (appropriately) dan+el: "judge from God" or Scots Gaelic, from "Domhnall" (world ruler).
Mallory, Elaine: The name Elaine is of Greek origin, and the meaning of Elaine is "sun ray; shining light". French variant of Helen. Surname footnote: Mallory is an English surname thought to be derived from a French word meaning "beautiful." Possible connections: One potential choice is (author) Thomas Malory's Elaine who acted as bearer of the Holy Grail. The same Elaine who tricks Lancelot into sleeping with her, trapping him in the role of her lover. (Citation needed.) (contributers: dimpwnc)
McCoy, Ebenezar: The name Ebenazar is of Hebrew origin and means "Stone of help." Surname footnote: McCoy is an Anglicisation of its Irish Gaelic form Mac Aodha, meaning "son of Aodh" (an old word for "Fire", a Celtic deity). Possible connection: The expression "The real McCoy", colloquially means "the genuine article." (Harry has used the idiom in the books - citation needed.) (contributers: Elegast, dimpwnc, and Serack.)
Here are the requested citations:
Example of "real mccoy" usage:
[Harry]:"Valmont duped the third party into taking a decoy. Then she grabbed the real McCoy and ran." (Ch. 14, Death Masks.)
I'm not sure what citations you want here; it's probably TMI, but it was fun skimming Morte. Feel free to pull out whatever you want, or leave it all here if it's too much speculative detail.
-Elaine of Astolat: Tennyson's "The mirror crack'd from side to side/the curse has come upon me cried/the Lady of Shalott." In Morte, she falls in love with Lancelot, cares for him when he is wounded, and, when he spurns her, dies of grief[XVIII,XX]. The physical description of Elaine Mallory and Elaine of Astolat are quite similar.
-Elaine of Corbenic, Pelles' daughter: bears the Holy Grail a few times[XI,II],[XII,I]. Falls in love with Lancelot and tricks him into sleeping with her--twice[XI,II-III],[XI,VIII]. Via an enchantment, he believes he is sleeping with Guinevere. Elaine has Lance's kid--Galahad [XI,III]. Lance goes mad with grief, and they stay together for a while [XII,IV-VI]; that's the end of her story in Morte. In Once and Future King, like Elaine of Astolat, she also commits suicide.
[edited to add] One question--are you only interested in etymologies, not in popular culture references? If the goal is actually finding JB's inspiration/allusions, pop culture refs are probably significant/useful. Some names also clearly are pop culture refs, e.g. the tiny tinkerbell-fairy Elidee->LED, the WOJ that explained the origin of Harry Dresden's name, and another WOJ I remember stumbling across that said that Billy and Georgia's names came from a TV program (I did a brief check and couldn't find it....Serack, as resident WOJ rockstar, do you remember it?)