On the Harry believing everything he's told tangent, I think it's mostly where he hears it from... He hates the Dursleys and has always hated them, so I doubt he'd believe bad things about his parents that they may have told him. But he'll believe something that he hears a respected older adult (like McGonagall or Dumbledore or another teacher) say usually without question, because it's just human nature to be more willing to agree with people we like.
And in Phoenix, he did at least make an effort to try to contact Sirius before running off to the Ministry to try to save him... So he got a little better xD (even if his checking in was at Hermione's persuasion...)
Oh, and to the gathering I'll be going as... Well, it's supposed to be Ginny, but everyone seems to think I'm Hermione, so it's kind of a mix between the two I guess xD