Which brings me back to my question: if Mab is Mother Winter's actual daughter (as said my Mama Summer in SK), who's the poor dumb sop that got it on with Baba Yaga?

We were told in a WoJ that the current Mab is the second Mab. If Jim is following the tradition of having 3 Baba Yagas, then it would follow that originally they were all sisters without the direct Mother -> Daughter relationship.

That said, though I haven't read it myself, I have seen people mention how the mantles shape a person to be a certain way (ie, Winter Lady not being allowed to have a child). If that is the case, then as Mother Winter, she probably had to have a child at some point; however, that doesn't necessarily mean she did it as Mother Winter or even as Baba Yaga - just that at some point it happened.