But Lasciel was always the temptress - that's what (Bob?) said from the beginning. And while Lash was an impression made on a mutable material and able to change, I would imagine that when we saw her in the beginning she was still mostly herself.
The difference between how we see her for the first time in Dead Beat versus Skin Game, I imagine, probably has more to do with the difference between the face she gives to the person she's seducing, compared to her being completely unmasked and filled with rage.
Mab is a great frenemy - it's pretty darn obvious when she's doing something perfectly awful, and Harry's caveman instincts are pretty easy to tell from his normal train of thought. With Lasciel in his head, the line would be grayed far more, and I think that Harry wouldn't be quite where he's at right now.
Ultimately, though, I don't think that I'd be happy with a Denarian Harry. This is going to be really controversial, but I felt like the Lash storyline ended really prematurely. I never felt like Lash herself was changing, and I felt like Harry kept creeping closer and closer to taking the coin. I mean, he has Lash possess his freaking tongue to talk several dead languages in front of allies. How more comfortable can you get? But then, Lash is suddenly all, "No one's ever lasted this long, clearly I'll never seduce you into taking the coin!" and Harry's all "Yeah, but you could go have a life of your own, why not?" Completely out of nowhere right before the final act of White Night.
The scene with Lash talking about the church and reminiscing about Heaven was beautiful, though.