1. check you're league, talk to the organizer, as every league operates differently.
2. 1, the only time you're allowed more then 1 deck is league play.
3. That depends on two thing's, you're national rank and weather or not you make top four at nat's. If you get in the top 2 you go to worlds for free.
It's actually better for you to attend a league and talk to an organizer, they'll tell you all this and more, there's various things that organized play offer's so talk to a TO. cya