Most of your reasoning is based on the dress, and I just think the rest of the evidence is far greater. The paintings could have been done at a stage in production before the intro video was done. Plus, I think the Little Sister is Eleanor BECAUSE of the blue dress. The blue of Eleanor's Little Sister dress matches with the blue of the butterflies in this scene and in the DNA picture. Blue butterflies in DNA = Eleanor, so blue butterflies surrounding Little Sister in blue dress = Little Sister in blue dress is Eleanor.