This is out there, and I admit probably not right.
Could Leah be a Mantle of Lara's?
I thought of this because Maggie was with the White Court for a while. Perhaps she was best friends with Lara, who when using the Leah Mantle swore to protect Harry. It may explain how Lara was able to get favors from Mab which has resulted in marrying Harry. Leah always wanted to own Harry, and in a way this is a way to be linked with him..
She may have also been the conduit for Nemesis who got both Maeve (through Leah), and Justine via Lara, if they were in fact the same person.
Like I said it's probably wrong but when I look back at Lara's possible connection to Maggie via the White Court, wanting to marry Harry, and everything it feels like it would be an interesting twist