... Somebody dropped a dime on Molly that put her in the juvenile justice system ...
Honestly, that could have been just "random" bad luck. Not *everything* has to be part of the Grand Conspiracy. Given how prevalent marijuana is in high-school, the odds that
somebody is carrying -- in a car-full of teens -- is awfully high.
But that provided an opportunity, I think, that was ruthlessly exploited... or, as you suggest, it could also have had a little nudging from the Spooky Side.
... She was manipulated into performing Black Magic. That does not appear to have been Mab...
I don't think we have *nearly* enough info on Molly's magical training/education before
Proven Guilty. There's nothing to indicate that anyone in particular was (or was not) involved. Mab could have been... Titania could have been... Ancient Mai could have been... <shrugs> Or it could have been 100% mortal stuff by low-level practitioners and wannabe's and untalented booksellers &c, without any push from any of the Supernatural side of things.
My bet is on Mab (possibly via one or more proxies). I think she was "nudged" toward mind-magic (and fear) specifically to get "on the same wavelength" with the phobophages, Mab's personal cadre of Fetches.
Too many elements "came together" in PG for me to think Mab wasn't manipulating Molly from long-before the beginning of that volume.