Mab was deliberately trying to draw out the attack and to have Harry witness the aftermath.

If you bear this in mind you can fit everything into place. Mab is a mistress of manipulation, she is manipulating both Harry and the Denarians, almost certainly with a view towards identifying the infected Denarians and who is behind them.

She has certainly drawn out Namshiel and probably Lucifer as being Nemesis infected, she also confirmed Maeve's infection.

Mab NEEDS Harry and to have Harry operating from his free will. She cannot command Harry, she can only persuade him, or manipulate him into thinking he has arrived at a particular point through his own decisions. We have yet to see why Mab needs Harry, doubtless it is linked into him being a Starborn.

I think most of the mysteries of Proven Guilty are in fact intended to get Harry to Arctis Tor at that particular time to witness the aftermath of the attack and the cure of Lea. That means Mab.

On approaching Arctis Tor Harry see the bones of the Defenders destroyed by much, much more fire than Harry could have produced, specifically Hellfire. Harry is a powerful Wizard and has an affinity for fire and is backed by a Denarian and couldn't have pulled this off. Thorned Namshiel most likely wasn't powerful enough either.

Only one being is at that power level and wields Hellfire. Lucifer. He is mentioned in PG "of course, I suppose someone looked down upon young Lucifer and considered what tremendous potential he contained" page 391 Orbit Edition. Did events get Harry thinking of Lucifer?

What could have drawn a Nemesis agent out to attack Arctis Tor? It doesn't weaken the Winter Court in defending the Gates . I think the reason was to release/kill the Nemesis infected Lea, but the attack failed to do either.

The Winter/Summer conflict was engineered by Mab to leave Arctis Tor relatively lightly defended, with Lea as bait. She used this to confirm Maeves infection (suspicions raised by Her use of Jenny Greenteeth against the Bordens against Faerie nature) and to draw out Lucifer.

In parallel she needed Dresden to witness events, Molly was the plan for that, abduct her and take her to Arctis Tor. Mab arranged for the Gatekeeper to tip Harry off, for the car accident to give her time to fix Little Chicago, fix Little Chicargo so Harry could pinpoint the black magic (only for that to be unnecessary due to Molly phoning him, poor Harry a concussion from Mab for nothing). Sandra Marling was most likely a tool of Mab, perhaps Ekdest Fetch? As the connector between Molly and the Con, and setting up Madrigal (who is also a suspected agent of the Black Council.

Why have Dresden witness this? Well he passed on the fact of the use of Hellfire at Arctis Tor to Nicodemus in Small Favour, which appears to have set the ball rolling on the events in Skin Game, and of course Lucifer was powering the traps for Marcone and the Archive.

In Changes Lucifer whispers to Dresden the words which provoke the attempt on his life to thwart Mab, breaking the 'rules' leading Mab to take extreme measures to save Dresden. Preservation of an eye witness to Arctis Tor? Revenge for breaking up Lucifer /Nic? Nic goes for the knife as it can kill Lucifer who Nic strongly suspects is Nemesis infected and changes the whole motivation of the Fall.

Oh and Mac witnessed Maeve in the pub, he's working for Mab, why else did he get neutral status, and get healed by her in Cold Days, no debt incurred.