Did Miss Emily not know Colonel Satoris had been dead ten years when she faced down the Alderman?
Miss Emily reminds me of Miss Havisham from Dickens's "Great Expectations" who tells Pip, "I know nothing of the days of the week or the months of the year." Perhaps, it is not so much her not knowing that Colonel Sartoris is deceased when Miss Emily tells the alderman who has come to collect the tax, "See Colonel Sartoris" as it is her rejection of the New South.
In another instance of her rejection of the New South, Miss Emily refuses "to allow them [workers] to fasten metal numbers above her door and attach a mailbox to it." Her remaining in the old days is, in fact, symbolized in the exposition as her house, once located on the "most select street" was the only one left, in ruins: "an eyesore."
(This house, too, reminds me of Miss Havisham's ruined mansion, much like the ruined lady.)
Miss Emily did not know Colonel Sartoris was dead, no. She had been out of touch for so long with the town that she was not aware of his death. She truly lived in her own little world and was out of touch with everything going on around her. She lived in a microcosm and was only in touch with this world, neglecting to address anything else outside of it.
She chose this path...to be out of touch with those around her. Unfortunately, this was ingrained in her from early on when her father kept her secluded from the rest of the world when she was growing up. She did not know how to socialize well or to entertain. She did what was common and safe for her!