Please don't take this wrong, but is there a reason we should care whether that's true or not? It seems to me that Snady is either a cunning evil drow who merged with a demon in order to gain advanced demonic abilities or is a cunning evil demon who is using a drow's body. I don't see why the distinction would matter, unless you just want to classify her for academic reasons or something. The greenie cameo person declared her dead, which somewhat supports her being dead at that moment, but maybe she can demonically control her body well enough to fake death.
My take is that she is, and already was, a Ver'drowendar, which I understand to be a drow/demon hybrid that has the Drow's personality in control. But anyway, to me, the potentially interesting thing is that it probably wouldn't matter, assuming her personality continues. Snad would still be Snad, whether she's a demon now or not and whether she already was a demon or not.
ADDED: One thing that might be kind of interesting was if the demon took over, but had a similar personality to Snady. In that case, her behavior might be different, but not that different. It might make for some interesting interactions. I can't imagine Kern pulling that off, though, and there isn't enough time left before the next timeskip for much of that, anyway.