^^'Yes' on the second and third things; 'sort of' on the first part.
Spoiler Below As I interpret it, Akira was just a normal psychic (feh) at first; however, his powers developed much faster than those of the other children in the government program, to the point that he attained the same sort of world-devouring power that Tetsuo eventually manifested. After the first disaster, the army dissected and froze Akira to prevent such power from being released again. Later, after Tetsuo thawed Akira's body parts out, Akira came back to life, took stock of the situation, and annihilated himself, Tetsuo, and the other children, seemingly in order to protect the world from Tetsuo and, perhaps, to save Tetsuo from himself. (That's how I see it, anyhow.)
Hell of a movie, by the way. Even if you don't understand everything that happens, it makes for a tremendous action flick. And the animation still looks great.