Having not read much Nabokov criticism yet, I am still mostly indebted to annotators online, and one astute contributor on genius, points out that that lemniscate "is a standard meteorological symbol for haze", connecting to the poet's desire for his daughter Hazel's soul to be eternal.
Haze generally is a phenomenon of obfuscation, a grey filter inhibiting vision and clarity, composed of dry particles such as smoke, dust, (stardust?) and sand (the latter a unit of time, a thousand years ago).
This also made me realise for the first time the obvious double use of Haze for the names of Hazel Shade and Dolores Haze.
Shade notes that all colours made him happy, even grey. Lolita's eyes, meanwhile, were grey.
Another annotator, commenting on these lines
A thousand years ago five minutes were
Equal to forty ounces of fine sand.
Outstare the stars. Infinite foretime and
Infinite aftertime: above your head.
They close like giant wings, and you are dead.
links them to the opening lines of Speak, Memory (which, unread, is glaring at me from the shelf)
"The cradle rocks above the abyss, and common sense tells us that our existence is but a brief crack of light between two eternities of darkness. Although the two are identical twins, man, as a rule, views the prenatal abyss with more calm than the one he is heading for (at some forty-five hundred heartbeats an hour)."
and notes, also, 'that an hourglass resembles a pair of wings.' (And a lemniscate, wouldn't you say?)
Really the lines from Time, As a Symptom by Joanna Newsom from her cyclical album on Space and Time, Divers (which I think draws some parallels to Ada) made me think of this concept for the first time, clearly echoing Nabokov's concern with how lightly we perceive the abyss of time preceding our lives:
And in an infinite regress
Tell me why is the pain of birth
Lighter borne than the pain of death?
The shape of the poem mimics the symmetrical wings of eternal time on either side flanking that ephemeral flash of existence, and also resembles the lemniscate, ∞
That flash, extrapolating from the form, must sit at the very center of the poem, falling somewhere, depending on whether we count the 'missing' 1000th line or not, within verses 499-500.
A blurry shape stepped off the reedy bank
Into a crackling, gulping swamp, and sank.
Considering the infinitesimally small amount of time Hazel Shade existed in the grand scale of things, her soul, effectively, is being born into existence and stepping off it at the same time, and so her leap off the bank can be considered to describe both events.
And the rest is rust and stardust.