I think the Denarians play in a different league from Gard. I also think while she make be acting as muscle for Marcone, she actually works for Vadderung and his best interests.
Ivy is full on Archive now, she has no boss, nor was she ever friends with Marcone as far as that goes.
Eldest Gruff knew about them. Maybe she's not on a first name basis with Tessa or Nic, but I don't think she could operate for very long without at least knowing "terrifying demons, run away" and "Potential Dennis The Mennis tactics, equally cursed, dodge small projectiles at all costs." I'm not sure if the Fallen can turn a valkyrie, but they managed a Genoskwa, so who knows.
I guess an Eldest Sidhe is maybe not the lowest bar I could set compared to Gard. But her job is also explicitly information... mongering? About the supernatural. I'm not sure if Marcone has a subscription deal, or if it's paid by the word. Maybe both, depending on the words.
LordDresden, I actually really, really like your hypothesis about the mirror image thing. I'm pretty much convinced. If not literal, perfect opposites in all things, then just super close thematic counterweights.
If you apply that to Marcone? In Mirror Mirror, and then his connection with the Archive, and then the Big Apocalyptic Trilogy? I think you can chart the rest of his course through the series.
I'll refrain from getting into the details, because now I gotta go construct a big, rambling WAG... tomorrow.
That's why I'm not addressing those other points, Mira. I will say that Marcone's friendship with Ivy was originally a misunderstanding. I just meant linked in that they could be traced to that event, and then maybe some thematics on top of that. But now I just got an idea for what they could be, and I want to iron out the idea before I go blabbing and confuse everyone. Mostly myself.