Depending on how you define magic the Denarian's use it constantly. The classic example is that they are immortal in either form. If you mean that they don't throw fireballs then you're right, but that is merely a nod to the fact that there are only so many ways to represent magic in prose.
I'm saying something at a lower level. Butcher made magic a use it or lose it affair. And he used it in a fairly prominent way. He justifies Molly's talent and Charity's lack of talent using this device. You can always say that Marcone had latent powers, but by canon you use them or lose them. Marcone isn't particularly old but he's closer to Charity's age then to Molly's. This at least suggests that Butcher is using deceit to further the story versus having a coherent vision. For theory crafters there are some other implications as well. Take Malcolm. The fact that Eb said that he was a mortal is now meaningless. Because if Harry could miss Marcone, Eb could have missed something similar about Malcolm.
Susan triggered the curse when Harry murdered her. This is one of those arbitrary points in the story where Butcher uses the distinction to keep Harry from using magic to murder directly. He can cut Susan's throat but he can't strike her down with lightening.
On Chichen Itza there is a two minute span where Harry blanks out. It's meaningful because it is otherwise unnecessary. I suppose this is what they call Chekov's Gun. I think ID Harry or alternate Harry may not be as squeamish as our Harry and gave the curse a hard push with whatever Butcher is concealing about what Harry is.
The effect of the curse is to explode the heart outward like something reached in and yanked it out.
I rather think that a ghostly immaterial idHarry spent two minutes teleporting all over the world
literally getting his hands dirty as he
literally executed the curse. A brief flash of a ghostly apparition, a yank, suddenly flashing to another victim- it would explain the nightmares and even some of the flames
et al if magically talented Red Court tried to ward themselves as they saw their compatriots fall.
Separately:
The primary purpose of the KotC is anti-Denarian, not their sole purpose. Murphy using Fidelaccius at Chicken Itza was authorized by the Sword, as evidenced by it pronouncing judgement using her against the Lords of the Outer Night.
If I am right in my theory that Harry is Uriel's Knight- the Knight of Vengeance/Retribution/Judgement- as Michael was Raphael's and Sanya is Michael's and Shiro was Gaebriel's- then it was two Knights ensuring that one of their brothers did not commit a mortal sin and descend into being a monster.
To be completely accurate, I think that what was left of Lash is Bonea. The "personality" took a mind bullet for Harry. The essence of shadow- the separate spiritual being-ness- Michael might be correct about as unkillable. I think
that regrew into Bonea, saved by the act of love that was Lash's self-sacrifice. WoJ was that we saw Lash in Ghost Story- the angel of death that was angry that Harry did not recognize her is a good fit.
EDIT: I also think this is why Mab and Uriel have been interacting. They are sharing an effective agent in a way that benefits both of them as long as Mab doesn't push for the more monstrous side of the Winter Knight- Uriel gets access to the Winter Knight capabilities for "free" and Mab gets a Knight of the Cross with Soulfire in exchange. She just needs to use other agents- like the Redcap or malks- for the less savory WK duties; quite a good deal for both of them.