Did she really see thinks in a more nuanced view or were you projecting that onto her? It would seem to me that at the end she reaffirms her belief in humanity. "It's not what they deserve, but what you believe" is a Certainty position. Nuanced would be reflective of Potentiality which I agree, would have made her more of a Changed character - and there is a bit of that when her world gets shattered...but she comes back.
As far as the Stop and the focus on the OS...Steadfast characters can sometimes be hard for newbies to a Dramatica understanding to grok so that's why the emphasis in the teaching on holding out for something to stop / holding out for something to start . The growth is still within the Main Character's point-of-view. In a very simplistic way you could say her growth is holding out on that Certainty until people stop telling her what she can and cannot do.
The Resolve and the "Protagonist's" Resolve in the OS are related through the Crucial Element. In a Steadfast MC story the focus will be on the work, or the Symptom and Response, so that's what you're seeing there in regards to Diana.
She sits on Probability, Steve sits on Possibility (See below) She's all about what will probably happen if no one goes in there and fights for what's right, Steve is all about impossibilities and limiting what's possible.
So in some regards you could think of her character arc as holding out for people to stop telling her the odds. (Steadfast+Stop+Probability).
Superhero movies aren't always Steadfast. Superhero movies is a Genre - in Dramatica, Genre has everything to do with the top Domain level. You'll find that most, if not all, superhero movies have the usual MC in Situation, OS in Activity arrangement of Throughlines (Batman Begins the only exception I can think of off-hand).