If Sandra Marling is someone's thrall, she would be what Bob called a fine thrall. Someone who doesn't realize they are being controlled by another being. Sandra Marlings actions are too complex to be an ordinary thrall like we saw in Blood Rites. So she might be in thrall to the Fomor, in thrall to someone in the White Court; like the lawyer in White Night, or to one of the Fae or possibly other supernatural entities, but not Mavra of the Black Court. (According to Bob, Mavra most likely can't create fine thralls as she lacks any shred of humanity or empathy. Of course Bob has been known to be wrong at times.)
However, when Sandra sought out Harry, because the police were questioning Molly, this demonstrated that she was either acting on her own initiative; and therefore not a thrall, but an independent agent, or whoever was controlling Sandra was on hand at the convention to monitor events and change Sandra's instructions on the fly, when needed.
If she is an independent agent, Sandra Marling could be a disguise, just as Harry disguised Thomas in Small Favor, just a much better one than Harry could do. Remember that Sandra organized Spattercon for over a year. I just can't see any major supernatural being doing this, they'd get some mortal to do it for them, and perhaps we're back to Sandra Marling being a fine thrall. However, a wizard who is mortal is only changing their name and perhaps their appearance from time time. That is not so difficult to imagine. So this doesn't rule out Kumori being Sandra. Then again, I can't rule out a fomor servitor, either. Another possibility is someone could have recently murdered the real Sandra Marling and taken her place, but that also seems unlikely for a supernatural player because they would have had to pretended to be Sandra for most of two or three days and know exactly how to run the convention, just for those moments when pretending to be Sandra might allow this being be able to tilt events one way or another. It's possible, but it seems a stretch to me.
In the notes to the Dresden Files RPG; which are semi-cannon, after the events in Proven Guilty, Harry has Billy look into Sandra Marling and she just disappears after the convention and there is no history of her before she is the convention's organizer. This suggests Sandra Marling was not a thrall. The only problem I have with her being Kumori is the time frame needed to organize Spattercon. Kumori was working with Cowl a year before Spattercon. Not only would Kumori have had to do some fast changing of jobs, she would have gone from being Cowl's apprentice or assistant, to directly working with the Black Council (or whatever you want to call the other side). And she would have not have performed any necromancy for about a year because she didn't set off Mouse's black magic detecting ability.
I'm leaning to Sandra Marling being a either a Fomor servitor or some other Black Court (or Circle) agent or ally we haven't met yet.