Mab has had her revenge on everyone she can find. The plan and attack that started in Grave Peril, is defeated, finally, in Cold Days. Nemesis ploy has been defeated. The Reds are all dead. That attack cost her a Knight and her daughter and crippled Lea.
In Proven Guilty, at her weakest, she fears an attack by Summer and has been attacked by the Phantom Menace. She is concealing her weakness from her enemies both real and imagined. She smiles at the end because Harry has done what she couldn't do, reveal what Summer would do were she to remove her troops from the border.
Lara is testing Mab's limits when she uses the fairies as lanterns.