I hate to single you out, but I keep saying this and nobody's satisfactorily answered yet;
The model doesn't look like Nariko. Nariko has substantially longer hair and a notably robust bosom. By comparison, the model is almost flat-chested and has short hair.
On top of that, there's a referential picture of Yorda. I've seen 3D artists work. They don't use reference material of completely unrelated subjects to craft models. If he was making a model of Nariko, that'd be a picture of Nariko.
Hair, chest-size, reference picture - all signs point to Yorda. So why do people think this is Nariko?
I'm a bit flabbergasted reading your argument. You're right that the character doesn't look exactly like Nariko, but she looks absolutely nothing like Yorda. Hair length, hair color, skin color, and even height are completely wrong. I have no explanation for why a picture from Ico is also displayed on the computer. It could be completely unrelated especially considering that we know the character designers have pictures and models of characters on their desks. You could argue that it just doesn't look like Yorda because the model is still being created but if you assume that then the model could be literally anybody.
The only thing I am certain of is regardless of whether or not that model is for Yorda, if she is indeed appearing in this game, there is a zero percent chance that it will be as a playable character. She's a character with no fighting or moveset potential whatsoever and just the idea of her fighting goes against everything her character even stands for.
Going from memory because nobody's reposted the pic yet, but I did think it looked reasonably like Yorda. Not perfect, but it was in progress, not a finished model. (I don't know anything about game character modeling, do you start with a generic model and tweak it into the shape you want?) Also her usual appearance colour-wise is rather washed-out since she has a faint glow. Maybe they'll apply that later, or maybe they're just going for a brighter colour scheme altogether. Perhaps a bit of both.
Also, I remember thinking that some of the smaller pictures also onscreen looked like ICO screenshots. Mostly based on colour scheme at that scale, but it seemed plausible.
(And I've said it before, and I'll say it again... I DON'T want to see Yorda in battle, even if she's just paired with Ico - she should be limited to assisting with super attacks. I don't want to have to fight her.)
--- RocketJess, blasting off again! PSN: iscah_rambles