How does the opening of Dances with Wolves depict Dunbar (& does it reveal whether he can be trusted with assumptions about Native Americans)?
The opening of Dances with Wolves is very bloody indeed: bloody and full of irony.
John Dunbar is wounded in battle during the Civil War. It is immediately obvious that he needs his leg amputated, but after seeing a soldier with only one leg on crutches (and, perhaps, hearing that there was no ether to dull the pain), Dunbar decides to ram a boot onto his bloody foot and end his own life. To do this, he takes a horse and rides directly in front of the southern army. He makes this ride twice, but he survives. The situational irony is, his sacrifice creates a distraction for the North to win that particular battle and Dunbar is awarded a medal for his bravery. The dramatic irony is that we as the audience know that Dunbar wasn't trying to be brave. He was, in fact, a coward.
In once sense, the movie is Dunbar's transformation from cowardliness to bravery. It takes his experience with the frontier and, of course, with the Sioux to teach him this. To answer your question, this is only the beginning of the story, so Dunbar is seen as a proud man (too proud to lose his leg) and determined (determined to lose his life instead). Although he is mistaken for a man of honor, he is not. However, it does make his superiors think he can be trusted with information.
The other irony about these opening scenes is that they reveal absolutely nothing about whether John Dunbar can truly be trusted with assumptions about Native Americans. Dances with Wolves is not really a movie about the Civil War at all. Instead, it is a movie about John Dunbar's journey from the "American way" back to the more honorable "Native American way." This is how John Dunbar both finds and achieves true bravery.