It's in ch.50 of CD
The "elemental" idea probably came from ch.48

"Demonreach was a genius loci, a nature spirit.  The fae were intimately connected to nature on a level that no one had ever been able to fully understand.  One could probably make an argument that Demonreach was one of the fae, or at least a very close neighbor." Cold Days page 473 Hard Cover.

Harry of course isn't a perfect source of information, and that doesn't discount Angel's being involved.  I wouldn't be surprised if an Archangel were powering it, with angel wards + Merlin using mortal magic + Sidhe magic all working together to bind the evil's that are there.

Upon rereading/listening to Proven Guilty, Lash describes the ward guarding the Carpenter's safe room as a working by Rafael or one of his lieutenants. I don't know that this entirely negates Mac as being Rafael, but I think it's food for thought. A nibble at least.

• Lasciel fell before Raphael would have transubstantiated and didn't recognize Mac for what he was. (seems unlikely)
• She started saying Raphael and then remembered he wouldn't be available so she added the leuitenant bit.
• She thinks Mac, even powerless, could have made the wards, and was in town to do so.
• These aren't the wards you're looking for.

First is your theory: I could be totally wrong, and Mac indeed used his Grace to help build Demonreach. "Raphael" would fit, at least in terms of the name's meaning, which literally means "God heals," (Hebrew origin) so that meshes well with the one supernatural ability Mac's demonstrated on the page. "McAnally" means "Son of the poor man" (Gaelic origin), if anyone's interested.

Danger Response - Earth Strategic Defense Emergency Network

DRESDEN

If you want mortals to protect themselves from the supernatural, you've got to get

P.I.T.C.H.F.O.R.K.
Paranormal Intelligence Tactical Combat Headquarters For Observation and Reality Keeping

The supernatural community has to watch out for those forkers. 

of course it does, he's the protagonist 🤣
Seriously though, Molly isn't an adult paving her own way, she has agency sure and no idea what to do with it. she's looking for a guide as it is, someone to show her an example of what she can do next. Harry is it, on top of the extraordinary amount of influence he has in everyone around him. An with Harry literally making the next choices on how things will play out, giving her specific options to run with. Some of those fates more heavily influenced of course. But the cheek scar has long been fabled about. (Recently feeling like knowledge has been lost here, or perhaps those who originally possessed it to spout are no longer here 🤔 ) Your view is very watsonian imo, but ignores the Doyalism of it all.

   One was an emaciated version of Molly, as
though she’d been starved or strung out on hard
drugs, her eyes aglow with an unpleasant, fey light.
One was her smiling and laughing, older and
comfortably heavier, children surrounding her. A
third faced me in a grey Warden’s cloak, though a
burn scar, almost a brand, marred the roundness
of her left cheek. Still another reflection was Molly
as she appeared now, though more secure, l
laughter dancing in her eyes. Another reflection
showed her at a desk, working. But the last…
   The last reflection of Molly wasn’t the girl. Oh, it
looked like Molly, externally. But the eyes gave it
away. They were flat as a reptile’s, empty. She wore
all black, including a black collar, and her hair had
been dyed to match. Though she looked like Molly,
like a human being, she was neither. She had
become something else entirely, something very,
very bad.