I am certain Lea is Harry's mother.

[Shift8]

At the ball in GP, Lea kisses thomas on the neck and it burns him.

Margaret made a deal with mab as part of her efforts to make harry starborn
.in return, she became a fae. This also explains why she is so keen on Harry and why she was he godmother from the beginning

[LostInTime]

WOJ is that Leanansidhe was Mab's Jenny Greenteeth and started out as a dew drop fairy or something similar. She grew in power and influence as Mab grew. Mab was Winter Lady and became Winter Queen in 1066-ish. So, Lea's been around a touch too long to be Margaret.

Found here. https://wordof.jim-butcher.com/index.php/word-of-jim-woj-compilation/woj-on-the-fae/

[The_Sibelis]

At the ball in GP, Lea kisses thomas on the neck and it burns him.

Margaret made a deal with mab as part of her efforts to make harry starborn
.in return, she became a fae. This also explains why she is so keen on Harry and why she was he godmother from the beginning

she'd just got done trading for a years worth of memories of Harry, spc from Susan who was falling or I should say, fell for him. She kissed Thomas to see if she indeed possessed true love. Thinking she thought it would protect her.
Lea probably drained Margaret for power or something. Whatever would have made Harry throw hands with her.

[Mira]

  Only problem with your theory is, don't Knights have to be mortal?  If Harry was half Fae he'd be a changeling and at least be able to chose his fate not unlike Sarissa.  No, I seriously doubt that Lea is Harry's mom.

[LostInTime]

Fix is an undeclared changeling and is the Summer Knight. Likewise, Lily was an undeclared changeling and Summer Knight and then Summer Lady.

[Mira]

Fix is an undeclared changeling and is the Summer Knight. Likewise, Lily was an undeclared changeling and Summer Knight and then Summer Lady.

Summer Court's rules may be a bit different from the Winter Court, not saying they are but they might be.  Also this, where is Malcolm in this equation?  We've seen pictures of pregnant Margaret, there is no dispute that Malcolm was Harry's father or that Eb is his grandfather..  So are you saying that moments after the baby was born, Lea switched her baby who then became Harry?  So in that case, who is Harry's father?  You think Malcolm wouldn't figure out that his wife wasn't his wife?

[Yuillegan]

I hope not, Harry has noticed Lea's "loveliness" too many times for that to not be weird. And as others have pointed out, she is much too old (unless you get into time travel stuff).

I read an interesting theory a while back that there is a reason Lea calls herself the Leanansidhe. Sidhe are the ruling Fae, the high born, the originals...not the intermixed with mortal ones that came later. It's a denotation of her higher origins. It's how the seemingly contradictory statements of A) "all Fae have mortal origins" and B)"Fae come from dew drop faeries like Toot-toot with enough belief and importance attached" are aligned. The theory states that some Fae (the Sidhe etc) are from dew drop Faeries origins and some are those bred/made of mortals.

So I don't think Lea is Harry's mother. Although it's an interesting idea!

[LostInTime]

This is an open window. Eb could have theoretically knocked boots with Leanansidhe, and fathered Margaret. Even though Eb's wife was mortal, nobody has ever said that Margaret was the daughter of Eb's wife.

Margaret may have, on her deathbed, declared for fae and be alive, somewhere in the Never Never.

I don't think it's likely. If Margaret had survived she'd have definitely taken Harry with her, if she were able. But it's a fascinating idea.