Well, here's the logical chain supporting the theory:
We have WoJ that Lasciel was in GS, but not referred to by name. The majority theory is that Lasciel was the whispering shadow, both because you can construct a logical set of motivations that would cause Lasciel to act in that way, and because there aren't any alternative appearances that make more sense. Therefore, for what follows, we are assuming that Lasciel was the whispering shadow.
Logically, if Lasciel was the whispering shadow, she would have to actually have the ability to whisper in Harry's head. Yes, this is obvious, but it's also important. If you go back and check the scene at the end of DM from Harry's first contact with Lasciel's coin up to the point where he encloses it in the circle and buries it in his basement, I seem to remember rather a lot of Lasciel talking in his head, even when he no longer had skin contact with the coin. This mental contact abruptly cut off when the warding circle closed around the coin.
(The counterargument would be that this was all proto-Lash, the shadow Lasciel had just stuck in Harry's head, and the whispering cut off as part of the shadow's long con, tricking Harry into thinking--wrongly--that his direct, whisper-link connection to Lasciel had been cut off by the effects of the warding circle. This doesn't work, however, because it would completely undercut the entire Lash plotline, and JB has been quite clear that the Lash plotline was legitimate--Harry did manage to change Lasciel's shadow into Lash. Therefore, we can conclude that Harry's ward was effective, and it did cut off direct contact between him and Lasciel.)
Now, if Harry's ward was effective, and Lasciel's shadow was able to transform into Lash in the absence of Lasciel's supervision, how was Lasciel later able to contact Harry again in Changes? If Salacia's theory at the beginning of this thread is correct, Lasciel's whispers began in TC, and continued until the missing scene in Changes that was revealed in GS. Logically, we must conclude that Lasciel's coin was no longer behind a working warding circle, potentially as far back as sometime during TC.
We know that Harry dug up the coin after Lash "died" in the Raith Deeps at the end of WN, and handed it over to Father Forthill for safekeeping. We don't know its exact movements from that point, but since Father Forthill has been KotC support for many years and knew very well the significance and danger of the coin, we should expect that he was extremely careful. On the other hand, Nicodemus has made a hobby of recovering coins from the Church for about 2000 years, with moderate success.
Nicodemus first learns that his plot to corrupt Harry using Lasciel's coin has completely failed at the end of SmF, when he tries to use Lasciel's shadow to defeat Harry just off the shore of Demonreach. Harry tricks him into thinking it worked for a moment, and then utterly horrifies Nicodemus by revealing that the shadow isn't there anymore. Harry works really hard at finishing him off right then, but we have WoJ that Nicodemus escaped, and we'll see him again in the book after CD.
So. You're Nicodemus at the end of SmF, and you've just discovered that Lasciel's coin did not do the job of removing Harry from his current mission--"saving the world, one act of random destruction at a time." We have no text or WoJ evidence to tell us what Nicodemus was up to after the end of SmF, so this is all purely conjecture. However, it would make sense for Nicodemus to do what he's done before, infiltrating the Church to recover one of the coins. If he did so, and was successful at removing Lasciel's coin from whatever warding circle held it, the timing makes perfect sense for Lasciel to start whispering to Harry again midway through TC, as in Salacia's theory.
To be sure, this chain of logic involves guesswork and supposition as far as the activities of Nicodemus and Lasciel. On the other hand, I don't think there's any suggestion in the books or WoJs that it's NOT right (please let me know if you think of any), and all of the actions taken in the theory are consistent with the known knowledge, motivations, and past practices of the entities involved, as far as I can tell.