I'm answering only your first question. A five-petaled flower of lilac is considered by us, Russians, a lucky tocken (normally, the flowers of which a lilac raceme consists have four petals). The painting by Serov mentioned in LATH by Vadim doesn't really exist and the girl herself seems a cross of several girls painted by Serov, one of whom is certainly The Girl with Peaches. There was an article by D. B. Johnson on VN and the art of painting (I don't remember its title) where he discusses that painting and perhaps even explains "the five-petaled lilac."

In literature, a character in K. Sluchevsky's long poem Prizrak ("The Ghost") "looks for happiness" in a raceme of lilac. May be there are other examples.

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