I recall that same theory that said Mother Winter and Mother Summer are actually two personas of the same Being. Like TWG has 3 personalities (the Holy Trinity), so it might be true.
I've checked out the name Atropos on the internet (I have to say that JB is damn good at research) and apparently she belong to the Fates or the Moirai (hello Greek Pantheon and Cold Days' foreshadowing to Skin Game Hades). She's a goddess of fate and of death. Skuld is the Norn version of Atropos. Basically the Winter Queen in both Viking and Greek mythologies.
I'd have to say that possibly Mother Summer's other names are Lachesis (Greek) and Veroandi (Norn). And if my logic serves me right, their most powerful Name(s) might be Uror (Norn), Clotho (Greek), or simply Fate in english. And Harry, by challenging them, was challenging Fate/Destiny. Which is sooo Harry.