As for it being justified - Marcone has been just as big as a threat since Storm Front.  Having soul gazed the man, Harry knows that Marcone could kill Harry and feel nothing.  Since sometime between Turn Coat and Changes, Harry has known that Marcone builds anti-Harry death traps.

But rather than kill the threat, Harry saves Marcone in Fool Moon.  Rather than react to the anti-Harry death trap, Harry arranges to meet the man on neutral territory.

[...]

Richard

Harry ressurect Sue's? I know she isn't human but, its necromancy anyway.

"Trixie," I said. "You can't possibly think that this is all right. Why are you doing this?"
 "I'm protecting what's mine, Larry," she said. "It's business."
 "Business?" I demanded. "Two people are dead already. Giselle and Jake were at death's door, and I don't even want to think about what would have happened to Inari if I weren't there. What the fuck do you think you're doing?"
 "I don't feel any need to explain myself to you."
 I blinked at her slowly and then said, "You don't know either. You don't know who he's marrying."
 She didn't say anything, but her eyes blazed with scorn and fury.
 I shook my head, continuing. "So you've just been eliminating all the women around Arturo Genosa. One at a time. You don't even know if you're killing the right person."
 "There's only one little girl toy left pretty enough to suit his tastes," she said.
 "Emma," I said.
 "And once she's gone, I won't have to worry about her stealing what's mine."
 I stared at her for a second. "Are you insane?" I said. "Do you think you'll get away with this?"
 "I'd love to see some prosecutor try me for witchcraft," she responded.
 Trixie was too stupid to believe me about the White Council and too self-absorbed to keep my name straight, but for crying out loud, she had to be human. "Hell's bells, Trixie. Emma's got kids."
 "So did Hitler," Trixie snapped.
 "No, he didn't," I said. "He had dogs."
 "Whatever," Trixie said.

[...]

 The adrenaline rushed through me, wild and mindless.
 I wanted to kill her.
 A lot.
 I hadn't ever felt that before-a sudden surge of fury, contempt, and disdain mixed in with a physical excitement only a few degrees short of actual arousal. It wasn't an emotion. It was nothing that tame and limited. It was a force, a dark and vast tide that picked me up and swept me along like a Styrofoam packing peanut. And I liked it.
 There was something in me that took a deep and gloating satisfaction in seeing my enemy on the floor and helpless. That part of me wanted to see her screaming. And then see her die screaming.

[...]

 I stared at Trixie for a hot, wild second, and the look choked her continued shrieks to whimpers. Trixie may have been female, but as of that moment she wasn't a woman anymore. She'd crossed a line. As far as I was concerned, she and her allies had forfeited their membership card to the humanity club when they killed Emma.

Firstly, Id! Harry (is the ! necessary? I saw you using it I think) said that he had been communicating with the shadow for a few months.

So in attempting to directly answer your questions, the best I can do is that the Shadow was constrained from offering true corrupting power to Harry but "cheated" by building rapport with his concious mind as Shela before he conciously accepted power (Hellfire) from her.  The pushing the envelope was only available to her because of his subconciously accepting the power (and his subconcious communicating with her in the time since), and she chose to pull that card dispite the consequences of "crossing the line" because she saw that he was undergoing some stressful situations that were likely to cause him to embrace the Hellfire she managed to offer his subconcious, and would thus give her lots of opportunities to offer him tantalizing power through the course of the events of DB. 
[...]
The thing is, the story goes through a lot of trouble to describe how there were lines and limits on what the shadow could do (Ref, the 2 dream sceens, 1 with Malcome, the other with the shadow and hot tub.  Interesting the symbolism in that in the Malcolm dream Harry was cold and uncomfortable, and Malcolm allowed him to mix his own Coffee to warm up... compared to Harry waking up in a luxurious hot tub he had never experienced before.)  Your Occam's Razor argument is not strong enough for me to just throw away all that world building effort.

To me, those two statements are completely contradictory...  My conceptualization of DV magic is the altering of reality with your will.  So in those terms Harry's empowering the circle was a willful alteration of Lasciel's ability to influence him.  Thus it was a sucessful magical spell, not an empty gesture that did absolutely nothing.

The true hurdle she had to overcome was not external to Harry.  It was Harry's own stuborness.  The singing is no evidence otherwise since it wasn't Lasciel/Lasciel's shadow doing it.  Rather the fact that the wispering was a constant preasure that required the singing to drown it out until the spell cut it off is evidence that the spell was an effective way of exercising this stuborness in a way that severly limited Lasciel's influence over him.

I don't think most bearers take that first step of rejection. They probably slip the coin in their pocket and carry it around like a secret treasure. Without that first rejection, they would be open to the full temptation at the start.

No, I think once you touch the coin the Fallen can try to tempt you. It is only following an act of refusal (Harry sealing the coin) that the Fallen is forced to resort to more subtle means.