It wouldn't surprise me. Jim often says he is a lazy writer...he has left that material in there for a reason. Could well be that she was part of the Black Council. I can't for certain say she was OG...simply because we don't really know how old the organization is, or even what it is. Not for nothing, but Kemmler (in A Fistful of Warlocks, 1800s) had a sort of proto organization of the Black Council but I believe it was the early Thule Society. Perhaps it's even older than that.
Margaret also is known to most of the supernatural community, particularly a lot of shady characters. The Erlking, Goodman Grey (possibly his father too), Nicodemus, Lea, Lord Raith, Arianna Ortega, possibly Cowl, quite possibly Mab and/or other Queens of Faerie, all the Senior Council members of the last 25 years, all the Wardens of the same time period (not the new ones that are not Morgan and Luccio's generation), probably a lot of the older Council, probably Vadderung, demons in Hell knew of her coming. Doesn't make her look particularly clean...until you compare her associations with Harry's and it's easier to see she might not have been so bad. Still though, Grey described her as a "peace of work" and he knew her. I doubt he would describe Harry the same way. Make of that what you will.
I doubt that Kumori is Margaret. Her manner with Dresden is more like an old friend, admirer, ex-lover etc. I certainly hope it isn't his mother (ew). But she doesn't treat him like a parent would a child anyway, even an estranged parent.