Yeah, I'd say that the cemetery one is Mavra. But there's no reason for them to get a count of six when there are seven places.
Maybe Harry'd initially been thinking that there were six
places to investigate, as he already knew how one of the seven locations had become marked by necromantic power? The jump from six unaccounted-for places to six necromancers was where the mistake cropped up, possibly from a worst-case-scenario fallacy: Harry knows he's in deep trouble, and unconsciously inflates that trouble in his mind.
Either that, or the process of revising that scene's text accidentally dropped a line in which Harry deduces that the ink spot for the graveyard is easily accounted for: it's where Grevane must've animated those
other zombies he'd brought with him to the Forensics Institute. (It's a pretty safe bet that he wouldn't drag a bunch of undead along on the Way to Chicago, where any passing Warden might notice them or their dark energies, and hauling them into town by car or train is even more conspicuous.) So, that gives two of the spots on Mort's map to Grevane, leaving five unaccounted for.