I like to think of the Dresdenverse soul as source of light.  A typical mortal's soul shines bright, and is the shape of their body, for lack of a better form.

Like Bob has described, mortals swap bits of their soul's constantly. When they hug, emote, get jiggy, or whatever else brings them close.

I think for beings like Mab, who started out with a mortal soul, their soul slowly fades over time.  Maybe not in size, but in brightness.  Like a light fading.  Eventually, like Butcher describes it, it can seem to completely dissipate, where its no longer there.

I think in Mab's case, and most of the Fae, they still retain their soul initially, but it fades.  But I think there's still enough there to interact with other souls around them.

In this case, Mab has had regular interaction with Sarissa for years, and I'm sure on some level, they've either shared or reflected each other's souls.  Mab has also had more contact with the mortal world through Harry, who we all know is big on soul.  And she's now bonded to Harry through the WK mantle ceremony, and the mantle itself.

So the idea that any remnant soul hasn't been affected by those around her seems unlikely.  It seems more likely that whatever soul she has left has been influenced by Sarissa, Harry, and any other mortals.  And since Sarissa and Harry both seem to be big softies, I'd say that's had a softening affect on Mab.

I don't know that she's gotten her soul 'back', so much as what she had left has been slightly rekindled by her relations with mortals.  I don't think she's going soft and fuzzy on us any time soon, but I'd say it allows her enough to remember the feelings she might once have had.  The light of her soul might still be dim, but it might be brighter than before.

Mab is nowhere near Uriel's power level.  Uriel has full Intellectus (which by definition includes the power to understand what you know) and was described by JB as "an Executive VP of Creation."  It would be a stretch to put Mother Summer and Mother Winter in Uriel's ballpark, and they are both clearly more potent than Mab.

Mab cannot have any sort of general form of Intellectus and simultaneously be unable to lie.  For example, when Marcone gets snatched by the Denarians and Harry asks Mab who took him, she responds, "I don't know."  It's possible for that to be a true statement (in a twisty Sidhe sense) if she believes she has partial knowledge; it is not possible for that to be a true statement from a being with Intellectus.

Which is another supplementary reason to believe the Scarecrow is working at cross-purposes to Mab.