The sidelocks are draped across the front of the torso. There's nothing within the scene which would plausibly
hold them in such a position or cause them to
return to this position when disturbed. It's conceivable that this design could work with a "pampered princess" archetype - a character who
avoids physical exertion and
constrains her movements in order to maintain her perfect coiffure. But your reference images clearly show that the character is
active and energetic.
So we're forced to either animate the strand with a
high rigidity coefficient (so that it re-creates the rest position shown in the reference image) or
deviate from the design by allowing the strand to rest near the shoulder.
We should be able to see dark outlines of hair strands within the rear mass of hair. But
we can't, because the original scene was poorly lit (the entire image has a limited dynamic range, and Hanabi's hair even moreso) and video compression has blurred out some of the subtle details.
Someone attempting to mod this hairstyle would basically need to
re-draw the strand edges from scratch.
The rear section of hair is just a big brown blob. We can't do anything interesting with it (animation-wise) because there's no available contrast. When a brown hair strand slides in front of another brown hair strand, the viewer won't notice any change. The animator would need to either
draw in extra details (so that minor animation effects would be visible), or simply
half-ass the animation parameters (e.g. rig the entire rear section as a single co-moving segment).
