But it does take her a long time to get to that point - she is so very innocent that until Othello becomes quite direct in his accusations, it simply never occurs to her that it could be anything other than a joke - adultery is utterly unthinkable to her.

That is exactly how I read her too -- I don't understand the people who take that scene where she's talking to Iago and the clown as an admission of guilt. To me, there's absolutely nothing in the text to support that.