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ok personally im in the “i think it is beta footage” camp, and it really does matter at all LOL but it was super fun to look at very closely as enrichment for my tiger cage lol. biggest thing for me are the eyes and mirrored details
the supposed beta (“new”) footage features eyes that look softer and more lifelike. interestingly the shader looks more or less the same (it uses parallax to make the pupils set into the eyeball, a feature to make it appear naturalistic/lifelike) but the shape/placement is diff
the in game footage edelgard tends to have eyes that look a little bulging/severe. the more we see of the top of the pupils the more it accents this look (whereas beta has more lid coming down over the pupil) tho from other scs i think there are some shape differences too?
all this to say: i can see this being intentional art direction for the game to her authority/character. the eyes are also a lighter purple which makes them pop even more.
another detail: the beta horns are rounder and imo closer to the original concept. it rly depends but i think sometimes i work on hero characters by getting them as close to concept as possible but as the game gets playtested more we make tweaks
sorry game dev headcanon time but creative + game directors who might not be involved w the character art process as directly may play the game and give feedback that takes in whole game context. ie can we make this character appear even More imposing
ok second thing from those same screenshots: the bangs framing her face are not mirrored in game! they are in the “beta” footage tho 
u might be like “who cares” but well. character artists do. they want to get away with mirroring as much as possible but sometimes when the mirroring is Too visible it starts to call attention to the textures being mirrored
