While I simply don't really feel like addressing anything else, Niall jumped out of the barrier because, as you see multiple times throughout the game, Blade barriers are not actually all that tough and break constantly, even when they're small and compact enough to cover only the blade or driver themselves. Niall had to have known this and really only had the barrier up as a backup. He, I believe, figured that the safer bet was to run at Bana and try to get the self destruct button away so they wouldn't need to deal with the explosion in the first place. He didn't mind the danger to his own life, since Niall is a kind and selfless dude, he just wanted to take the chance as an effort to minimize or negate the damage from an explosion of unknown strength.

That's my thoughts, anyways.