To everyone above after half-reading what they said:
Ok, here's a thought.
Maybe they all have the scar. Every time we saw April, she had her bangs covering her right-hand side. Implying she might have a scar, but not necessarily. We know Hannah does have a scar, certainly. Hannah also appears to be an artist.
Oh, oh oh. Also - Saga believes the first time she walked was when she was 7, but it was actually before that - she was about 4 or 5 I think. There's an implication that she did it the first time we saw her. Magnus doesn't want her to leave because Etta didn't come back the last time she left, to do something very important, which no one said what it was. (He is kind of being selfish but it's understandable)
It does make sense if when an incarnation of Spring dies it goes back to the House of All Worlds. Hannah would therefore be the 2nd time that Saga walked. If Hannah is not a dreamer for real (Abnaxus said Zoe was the only one) then she would have memories of Mercuria kind of like she was *shifting* and not dreaming. Now, if Etta was actually the white lady, then when Brian killed her, because he's one with the undreaming (if not replaced) that's when she didn't return back to Magnus and Saga.
"People assume that time is a strict progression of cause to effect, but *actually* from a non-linear, non-subjective viewpoint - it's more like a big ball of wibbly wobbly... time-y wimey... stuff."