As I see it, the weakness in your answer is that it seems common knowledge (the Knights, Nic) that nobody ever resists a shadow more than a few weeks before taking up a coin, yet Harry seems to have had no such issue/conflict between DM and DB (about one whole year). If Lash was able to, you'd think she would have started building "Sheila's" persona a whole lot earlier - to use at the appropriate moment.
Look at the type of people the Denarians usually go for:
"Tessa chooses their hosts from among the downtrodden, the desperate, those who believe that they have nothing to lose. Those who will succumb to temptation the most rapidly."
I grunted. “Lot of those around in the wake of a big nasty plague. Or any kind of similar chaos.”
“Yes. We believe that it is one reason she collaborates with Nicodemus from time to time.”
“She’s focused on short-term,” I said, getting it. “He’s all about the long view.”
“Exactly,” Michael said. “When he threw Lasciel’s coin at my son, it was a calculated gesture.”
“Calculated to rope me in,” I said.
“You,” Michael said, “or my son.”
That's bound to skew the average considerably. Look at Nicodemus' reaction in the same book:
“Oh, I picked the right coin for you.” He started to walk in a slow circle around me, the way you might a car at the dealership. “There are rumors that a certain Warden has been flinging Hellfire at his foes. How do you like it?”
“I’d like it better if it came in Pine Fresh and New Car instead of only Rotting Egg,” I said.
Nicodemus completed his circuit of me and arched an eyebrow. “You haven’t taken up the coin.”
“I would, but it’s in my piggybank,” I said, “and I can’t break the piggy, obviously. He’s too cute.”
“Lasciel’s shadow must be slipping,” Nicodemus said, shaking his head. “It has had years to reason with you, and still you refuse our gifts.”
“What with the curly little tail and the big, sad brown eyes,” I said, as if he hadn’t said anything.
One of his heels hit the ground with unnecessary force, and he stopped walking. He inhaled through his nose and out again. “Definitely the proper coin for you.”
On the one hand, he acts somewhat surprised that Lasciel's shadow hasn't managed to tempt him yet. But it's an "huh, that's odd" level of surprise not a "that's impossible" level of surprise (such as he'll show just a little later in the book). We know/believe Nicodemus thinks long term. He naturally expects that even if Harry picks up the coin, he won't instantly grow a goatee, but rather try to keep it hidden from his allies as long as possible. He never bothered looking for Harry again after giving him the coin, so clearly, his plans did not require/expect Harry taking up Lasciel right this instant.
I suspect Lasciel's Shadow was smart enough to realize she had no real hurry to corrupt Harry, and knew that if she wasn't subtle enough about it, Michael and Co. would do everything in their power to oppose her. Discretion was the better part of valor, that's all. Heck, we know/believe Lasciel and Anduriel don't get along. From Nicodemus' reaction above I would hazard the thought Nick thought Lasciel was taking her time just to be contrary (just like Harry is).
To Harry everything is a spell. (The DFRPG and I think WoJ refer to this as, "If all you have is a hammer every problem looks like a nail") Don't forget how the books pontificate on the amount of magic in a baby's laugh, and an entire stadium shouting "HAY!" after that silly tune. In otherwords, you may be putting too much emphasis on Harry's training to interpret my claims about his isolating himself from the coin's influence with magic. From certain perspectives there probably is something magical about Sanya throwing his coin into a canal.
Hm, let's use another example that's also from the books: what Harry calls "Old World Rules". Let's pick one specific one: keeping your promises. We know that Fae have to keep their word. We know that a wizard who makes a promise upon his power has to keep his word, or his own magic will attack him. Someone like Lara Raith, on the other hand, is free to go against her word. However, she never does, and whenever Harry suggests to her that she could, she looks at him as though he suddenly started talking an alien language. She's not constrained from breaking her word by any power or magic like the Fae are, but her own upbringing / self-interest make the idea basically impossible.
I want to believe the Dresdenverse is the kind of place where Sanya bears the Sword of Hope because he's the kind of guy that laughs when Rosanna tries to tempt him once again. I don't like thinking that the Dresdenverse is the kind of place where the Sword of Hope needs to protect Sanya from Rosanna's new temptation (I'm willing to accept a world that's a little of both, of course; shades of gray ftw).
A world where Lasciel's shadow is constrained by Harry enclosing the coin in a circle in the same way a Fae is constrained to keep its word has less appeal to me than a world where Harry enclosing the coin in a circle constrains Lasciel's Shadow the same way Lara Raith is constrained to keep her word.
Thrice I ask and done: which is the one your theory is describing?
(I admit, I've been wanting to say that for a while)
