There may be many recognizable words in Swinburne’s poem “Dolores” (addressed to actress ADA) to be found scattered in VN’s works.  There is one mention of Libitina in Pale Fire when Gradus moves towards Lavender’s Villa: “ He crossed it, walked through a wicket and up a curving gravel path, and found himself in front of Lavender’s villa. Its name, Libitina, was displayed in cursive script above one of the barred north windows, with its letters made of black wire and the dot over each of the three i’s cleverly mimicked by the tarred head of a chalk-coated nail driven into the white façade. This device, and the north-facing window grates, Gradus had observed in Swiss villas before, but immunity to classical allusion deprived him of the pleasure he might have derived from the tribute that Lavender’s macabre joviality had paid the Roman goddess of corpses and tombs.”  There are also the cigarettes inKQK named “Libidettes”…

 

Googling I came to an article (1978) that discusses death/Gradus. One of the included information is worth retrieving again, Mary McCarthy’s point relating Gradus and Pope.  There is another item on Horace.

Mary McCarthy points out that gradus is Russian for degree, and thus refers to a couplet by Pope, "Virtuous and vicious ev'ry Man must be,/Few in th'extreme, but all in the degree," in a passage of the Essay on
Man (11.221-32) which states one of the major themes of the novel.

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Horace (I.iii. 17-20), when the rhetorical question is asked, what could ever frighten the hero who first sailed the seas?
quem mortis timuit gradum
qui siccis oculis monstra natantia,
qui vidit mare turbidum et
infamis scopulos Acroceraunia?
(What step of death did he fear, the man who, dry-eyed,
saw the monsters that swim, the water storm-toned, and
the dreaded Acroceraunian rocks?).
J. B. Leishman translates the first of these lines as "what form of death's approach did that man fear . . . ?"⁷ Joseph Clancy translates it, "What could he fear in death's footstep . . . ?"⁸ E. C. Wickham interprets gradus mortis as "the step, the footfall of death," but adds that the Scholiasts took Horace's rhetorical question as meaning "'what degree of death? i.e. what death is so terrible that he feared it if he feared not the sea?" Thus gradus is either footstep or degree, approach of death or kind of death./ It is characteristic of Nabokov to make sport of a word that is untranslatable or has been mistranslated; in Pale Fire he also jests with Libitina/Lubentina, "funereal, con­cerned with corpses ...
Pale Fire  Journal article by Vladimir Nabokov; The Explicator, Vol. 36, 1978  www.questia.com/PM.qst?a=o&se=gglsc&d=95785070