Starting from the assumption that Scarecrow really was outsider-powered, I submit a slightly modified version of what happened that might fit the events we see:
Grave Peril1) Cowl et. al (Black Court) are preparing for a Darkhallow. Problem is that any of the existing major power groups would try to stop him from achieving their goal -- nobody wants an extra necro-god (and we have a WoJ that Cowl+Darkhallow could beat Mab) messing up the existing balance of power.
2) Thus, the plan is to attempt to distract all the major power groups -- in any way possible. A war is fostered between the White Council and the Reds, and the Athame is given to Lea to cause imbalances between the courts.
Summer Knight3) Initially, everything seems to work. Giving Lea the Athame caused Aurora to look for other sources of power - resulting in her going crazy. Lea herself gets infected. The White Council - Red Court war begins. Harry manages to take out Aurora, but the overall plan is still going well.
Dead Beat4) Everything is in readiness for the Darkhallow. The Reds are convinced to make a major attack on the White Council on the night before Halloween, and Lea tries something (rebellion?) that forces Mab to imprison her.
Option 1 as suggested by Thork, this rebellion might have actually been assault on Arctis Tor itself, preserved until PG through time differences in Faerie.
5) Unfortunately, the Darkhallow fails, thanks to the efforts of one Harry Dresden. This causes a *big* problem for the Black Council, because Mab now has the athame. Originally nobody in the Black Council worried about this issue, because they expected to have a Necro-God on their side. Now however, Mab has a major artifact in her hands, is healing Lea, and is certainly planning retaliation.
Option 2Lea+Athame = Minor imbalance in the Courts => Faerie Courts distracted.
Mab+Athame = MAJOR imbalance in the Courts => Big trouble.
In this option, the assault on Arctis Tor happens some time between Dead Beat and PG, and is specifically a raid to get the Athame out of Mab's hands.
Option 3As above, except the assault was made in order to free Lea.
In any case, we have a WoJ pretty much stating outright that the assault failed.
Proven GuiltyThe Black Court is now getting desperate. Whether they need to free Lea before she gets cured, or whether they need to get their hands back on the Athame is irrelevant. Time is running out for them.
Thus, they concoct a plan to summon some fetches to Earth and infect one or more of them. Once the infected Fetch gets back to Arctis Tor, it can either release Lea or maybe just get the Athame back.
How do you get a Fetch to come to Earth? Well, you need to summon it with a load of fear. Enter Madrigal, puppet of the Black Council. As Madrigal says, he was invited to come to the convention a year previous. ...Right around the time the Darkhallow failed?
On page 302 of the paperback Harry says the following:
"Whoever called up the phages," I said, "needed a way to guide them from the Nevernever to the physical world. They needed a beacon, someone who would resonate with a sympathetic vibe. Someone who, like the phages, wanted to make people fell fear."
Harry is talking about Molly, but Madrigal sure fits the description really well.
6) The Black Court plans the convention and summons a bunch of Fetches to home in on Madrigal. Glau is the cutout who is supposed to make sure Madrigal is in the right place. They manage to infect one of the fetches -- Scarecrow.
Glau is then killed by the outsider-powered fetch -- nothing personal, just killing all the witnesses.
7) Mab however has other ideas. She manages to get Molly to use fear magic, thus sending the (other?) fetches after Molly, instead of Madrigal. This ensures that Harry Dresden (AKA "known powers over Outsiders") comes into the picture. Note that this point would seem to make Sandra Marlin an agent of Mab's, rather than an agent of the Black Court.
8 ) Harry chases after Molly, and (as Mab planned) manages to beat the Outsider-infused Scarecrow right before Scarecrow manages to release Lea/Athame/whatever.
Note: If Aurora was also outsider infected this would make an interesting symmetry, as both Faerie Courts would have then employed Harry to excise an Outsider-based contamination.
9) In the process, Harry accidentally injures Mab by pouring Summer-Fire into the wellspring of Winter. This is
not as Mab planned -- fitting in nicely with the WoJ we have.
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