As
@liszst said: This movie is probably based on Gleen's life (since the story was written by a "Cleed Schreiber" and his name is "Cleed", he came from a religious family and everything, and the fact that he does not like the main character is also a hint of that), and the director "Jonas Bleibtreu" is probably the guy he was waiting for in the bar where he meets Edd, and whom he was in love with before and had a relationship, as the movie credits "Those Trivial Stories We Shared in Bed"...
About Chang... I think
@Despada is right, He really wants to dominate Edd somehow, because they don't have a "sibling relationship" anymore (I don't know why oõ... What happened between them?), so if he pays what he owes, they will probably never see each other again, and he wants to avoid it by increasing his debts, and if he can't afford it, make him work for him, so he can keep him all the time.