1. There are two possibilities: Sprouting annihilates a soul; or the soul remains powerless in the Land of the Dead forever.
2. What happens is left a mystery. Salvador calls the Ninth Underworld 'heaven', and getting there quickly is supposed to be a reward. However, in Aztec mythology the soul ceases to exist once it completes its journey.
3. In Aztec mythology the universe consists of nine levels. It's basically the same as any other type of layered cosmos: the divine realm at the top, then stars and planets, then the mortal world, and the realm of death at the bottom.