I'm not in the band (obviously), but I can answer some of these.
So far we've heard two B-Sides and one Acoustic. Bury Me was an iTunes pre-order bonus, You Never Noticed Me was a Japan Bonus Track as well as a bonus track if you purchase the CD through
http://www.pacsun.com/saosin, and Changing (acoustic) was also an iTunes pre-order bonus. All three can be heard by searching them on YouTube, but I'm sure will be available in the future (be it iTunes or the inevitable Re-Issue CD)
As for Japan getting a bonus track, there are two reasons. One is because import CDs cost more, so it's an added incentive to spend the extra money, and another is a way of saying, "hey sorry we don't get to play shows in your country all that often, hope this song softens the blow a little bit."
Now whether or not all 18 got recorded and there are 3 more waiting for us to hear, only the band can say.