I've always thought that Newtypes would be judged by Newtype abilities rather than their physical strength, so it doesn't matter about all the fights that they've gotten into and lost or won.
That being said, it wasn't really Amuro exactly who pushed back the asteroid. The combined... I don't know what to call it... feelings? thoughts? determination?... of all the pilots who wanted to push back Axis caused the psychoframes on the Nu Gundam, and I assume maybe the Sazabi's cockpit too to overload.
As for Seabook, I think there's some evidence that he might not be a Newtype at all. The biocomputer on the F91 reads data from the battlefield and feeds it into the pilot's mind, letting ordinary pilots "sense" things in a Newtype-y way. Now obviously there wasn't time during the movie to develop it but Seabook shows up again in Crossbone Gundam, and just sort of is shown as a really stellar ace pilot than anything else. Also when Berah's cousin appears and reveals that she "collects" Newtypes, she's interested only in Tobia, making no mention of wanting or ever wanting Seabook.
Banagher was menioned too, but again it's not really him so much as the Gundam he pilots. In fact from the 3 episodes out so far I kind of get the sense that he might even be a Cyber Newtype rather than a real one: all those weird flashbacks of him in that weird chair with that metal eye patch thing, and Full Frontal's comment that it takes an enhanced human being to use the NT-D (though presumably a Newtype could do it too, it's just that he chooses to bring up Cyber Newtypes only). Anyway from his description of the NT-D it seems like Banagher isn't actually the one that does all those crazy things the Unicorn can do, like stealing funnels and what not.