@Ghost: Yuki's explanation confuses most viewers and Asahina's information is limited therefore her explanation is incomplete. Kyon was looking for a direct simple answer and Koizumi is the only person who gave him one. He could only give out an assumption and not actual facts though but atleast it easily summarizes the ideas from all 3 factions.
Koizumi is a person who loves theories (I got a similar co-worker so I know how this guy thinks) and people like him would always like to make new theories and check them.
Kyon never really did completely accepted the explanation from the 3 of them (As per Analogical ending of season 1). He didn't see Haruhi the same way as they did. To him, she was just Haruhi. An mysterious girl who loves mysterious things that makes mysterious things happen without knowing them. He never did completely believed Koizumi's answer.
As for the viewers believing it. Well as I mentioned. This was the closest answer that we have on what Haruhi is. Kinda like the default choice at the moment until we get more facts.
As for Koizumi's manipulative behaviour. The only person I see that he manipulated properly is Haruhi.
Though I think Koizumi mentioned somewhere that he never really believed that Haruhi is God and he believes that its someone else.