After rereading I see what you were trying to say. And I'd argue that Mab doesn't enforce the Accords at all, she just wrote them. She's as beholden to them as any other signer. The enforcers of the Accords are every other signer. Kinda like how the UN is governed by the body of countries and not a single one, if one country acts up they all smack them down.
That's not entirely true. While the signees on the Accords do typically settle things amongst themselves (a la Harry's duel with Paolo Ortega in Death Masks to settle the dispute between the White Council and the Red Court), Mab does seem to enforce the Accords in some manner. In
Small Favor Nicodemus breaks the Accords by kidnapping Marcone (ultimately with the goal of snatching The Archive). While Mab doesn't directly intervene in the matter, she does act through Harry to resolve the issue immediately, as well as later on in
Skin Game when
we (and Nicodemus) learn that you really don't want to break Mab's rules or make her angry. Otherwise she'll play the long con on you, make you kill the only person you love, kick you off of the Accords, and leave you blacklisted in the supernatural community.
.And you definitely don't want to do anything to break the rules on Accorded Neutral Grounds or she WILL show up. (edit - looks like PirateJack beat me to it on this one, but you get the picture)
In regards to your actual campaign idea I like it. I'm running a game set in Pittsburgh and I had a similar idea of it having a powerful magical threshold around the city proper due to it being surrounded by rivers. Like
PirateJack said I think you could just say that St. Louis is a metaphysical crossroads and that could be the reason why the veil is weak.
Maybe instead of there being a giant magic circle surrounding the city, perhaps a leyline used to run right through the area and the St. Louis Arch was built to run along the leyline, taking the energy in one end and dispersing it over the arch to the other side and leaving the ground beneath magic-free? Perhaps you could think about introducing the Fomor into the mix in some way? Certainly with St. Louis being right along the water and a power vacuum popping up I'm sure they would want to be right in the thick of things.