Given Raith's protection from magic, and the fact that her death curse is apparently anchored to her children, I imagine it was more complicated than Cassius' curse, which wasn't very potent and only marginally effective, given that Harry had both Mab (the presumed whisperer) and Bonnie with him when he died.
Well, kind of what I was thinking was that using a birth to power some kind of spell was required for the Raith deal. We assume it was her death curse, but what if she used Harry's birth to anchor this reverse-bloodline-curse that she leveled against Papa Raith? I've even toyed with the idea that the thing was anchored at both ends; as long as the curse was in effect (i.e., Raith is still alive and hungry), Harry and Thomas (and by extension, Maggie and Ebenezer) were offered some kind of protection. It could have been what kept Harry hidden from Raith all those years; the old King would certainly have wanted Harry dead as soon as he realized what was done to him, rather than wait, you know, thirty-something years.
As for the Coin, I don't know about Lasciel. She knew
something about her and what took place with the White Court, but we don't know that she knows anything more than the rest of the Nickelheads Nicodemus keeps with him—Nic mentions that he knew and respected Margaret in Death Masks over his creepy-calm torture breakfast. Namshiel is interesting, and I got the feeling that Thorny goes through hosts relatively quickly, considering that he's never seen outside his shapeshifted form. I don't think that Margaret would have accepted a Coin from something that was so dominating, however. Lasciel is a better fit for her personality.
Against that, however, I absolutely believe that Lasciel would have started talking smack about Harry's mom in Hades's Vault, though. She wouldn't have been able to help herself, and nothing would have cut Harry more than this exchange:
"No one's ever turned me down before, Harry. Not once.
Not even your mother."
I'd imagine Harry would have gone into a miniature mental tailspin from that, and Lasciel would've wanted to twist the knife—so would Ascher.
Rosanna is a possibility; even the Poor Demon Girl act would have been compatible with what we know about Margaret's personality (as in, I could see Margaret pulling it off).
Still, Namshiel's possible infection makes him a strong candidate; I believe Margaret's overriding goal for a long time was stopping the Outsiders, and putting a Denarian in the line of fire of Nemesis is too tantalizing to ignore for me. I just don't think I could reconcile that with Margaret's personality.
