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.