a) demo mode is merely a trial, just like most mmorpg play for free first 14 days! when you buy credit you will continue your game and it will deduct credit on every cycle. Refer to the manual: http://www.airwaysim.com/game/Manual/General/Credits/
b) if you click change game under dashboard in game you will see the following:
for billing information just click dashboard or game area and it should be on your right side.
c) our master has answered that
basically each game is isolated.
My best advise(if you decide to continue on the game) is to apply for a mentor, depends on your desire to learn having a mentor may make your experience in aws much more enjoyable.
Welcome to AWS and good luck!