The young girls's weapon is just a weapon (though its pretty awesome). She has an insane amount of power that you get to see in the arrancar arc.

Heh, you were close Rasler. Kurotsuchi Mayuri replaced Urahara Kisuke (written the Japanese style) as the captain of squad 12. 13 is headed by Ukitake, the white haired captain with TB.

Yes, Urahara was exiled for a particular invention of his that becomes important later on.