Easiest one first. The Fomor are from everywhere in the oceans. What makes you say their main base is in Lake Michigan? Did I miss something.
Marcone survived most of his encounters with the supernatural, at least once the books start, because he is in the same city as Harry. Victor Sells would have likely killed him if not for Harry. Ditto all the werewolves. Maybe Bianca. Easily could have died from a plague centered on Chicago. I'm not sure how many would have died, immediately, from the Dark Hollow, but Marcone might have been one of them, he definitely would have his power diminished if the killing of Boney Tony went unanswered or he took on Grevane, his relationship with Harry is what gets him on the Accords in
White Night, if you can't figure out how Harry helps Marcone in
Small Favor, that's on you,
Cold Days would have killed everyone in Chicago, and judicious use of Harry helps him elevate his position in
Skin Game and then defend it once more in
Jury Duty.
The point is, it's not a coincidence that the first wizard to advertise in the yellow pages and the first vanilla mortal to get on the Accords live in the same city. It's causal. Additionally, if Marcone was to maintain his level of threat to Harry, he had to level up with him. His relationship with Monoc Securities was how he did that. He may well have only been able to obtain that relationship because Harry was in Chicago, and Odin wanted to keep tabs on Harry. Vadderung himself suggests that.
Someone with enough foresight might, for example, arrange to be in a position to assist a hotheaded young wizard of the White Council one day. Perhaps who I am is directly responsible for why I am here.
Harry signs for Marcone. Vadderung signs because Marcone has a relationship with Marcone because of Harry. Lara signs because Marcone saved her bacon because of Harry. Marcone is on the Accords because of Harry. No coincidence.