In the text about Pale Fire’s “Gradus” and death (quoted below again) the author mentions Kinbote’s equivocal use of “libitina” (in Swinburne the use is clear and unambiguous, it follows the line of Kinbote’s own explanation: goddess of corpses …)” :

“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 ...”  Vladimir Nabokov; The Explicator, Vol. 36, 1978  www.questia.com/PM.qst?a=o&se=gglsc&d=95785070

[QUERY] Libitina is related to death, Lubentina to life (Venus,eros, libido, pleasure). Lubentina also indicates a butterfly.

These parallels are fascinating but… is it justified to suppose that “Lubentina” has been purposefully, slyly referred to… “ad lib”? That it’s an authorial jest?

All the indicators suggest that it was deliberately chosen by VN, so this item has to be extricated from the text, explained, added to the list of authorial ploys. In Swinburne, the irony is there for those in the know: it adds to its “Dolores,” but it can be passed over, with no harm to an overall understanding of the poem (I mean the oscillation bt. life and libido, and death which are already an integral element of the poem).  

Could this collection of authorial traps be taken as an  instance of “unreliability of the author”, not of the narrator?