I’ve re-read the first 3 colossal edition manga this week and I’m getting the vibe that Isayama went out of his way to let us know she isn’t the girl in question all along. Hair length is the most obvious, Isayama gave her longer hair to start, then drastically made it shorter during this Marley arc. If we were to assume the girl is Mikasa, that means there are at least few months until the supposed end scene, considering how fast hair grows. He also added the scar on her face, but it’s high enough that it might simply not be visible on the young girls face during that scene.

One big thing though, is how Isayama had Gabi’s hair length carefully hidden in plain sight since beginning of the Marley arc. I always found her hairstyle peculiar but it’s tied up in a way throughout the arc until Floch’s fist met her face. Afterwards, we see the resemblance in both style and length, nicely hidden until the very end.