What I've been trying to get at is that the Lasciel-as-whispering-shadow theory (which I think is true) has consequences as far as the plot goes.
1) It means that Lasciel's coin is not in a secure location right now--middle of Changes through the end of GS.
2) If Salacia's reasoning is correct, and I found it quite convincing, Lasciel's coin hasn't been in a secure location for some time--since TC.
3) As best we can tell, neither Harry nor Father Forthill know this is the case! They think that wherever Father Forthill put the coin is still secure.
So the question is, did Father Forthill merely fail to ward the coin sufficiently (doubtful, given his position as KotC support), or has someone stolen the coin or compromised the wards such that Lasciel can now influence Harry? And if so, who did it?
By the way, if Salacia is right that "Because you need your brother to be all right" was the first whisper from Lasciel, that would support a Nicodemus-stole-Lasciel's-coin theory sometime between the end of SmF and the middle of TC. Remember, Nicodemus finds out that the Denarian hole card of Lasciel's shadow in Harry's brain doesn't work at the end of SmF. Since Nicodemus is terrified of Harry (WoJ) and his Lasciel-plot has clearly gone awry, he'd be very motivated to recover the coin and we know he has extensive experience in recovering coins from Church custody.
That doesn't mean that Nicodemus/Anduriel would know about Lasciel's revenge plot against Harry. At the end of SmF, Harry told Nicodemus, "Lasciel's shadow doesn't live here anymore." We find out later that Harry was wrong about this, but it was what Harry believed at the time, and I'm quite sure Nicodemus was able to note Harry's sincerity. I think it's pretty likely that if Harry actually had found a way to permanently remove Lasciel's shadow, that would have the effect of breaking his connection to Lasciel's coin, possibly by severing the principle of contagion permitting Lasciel's whispering communication. If Nicodemus compromised Church security (...again...) and stole the coin, Lasciel may well have decided not to tell Nicodemus that her line of communication with Harry was back open--we're told that Lasciel and Anduriel generally don't work together.