Re: The Joker's background?
About the Joker's background... I don't think that there was a single event that transformed him. It wasn't like he was a completely normal man one day and then *snap* he gets the scars and he becomes a crazy criminal mastermind out of the blue. Human mind doesn't work like that.
I imagine the Joker as someone who's had a very tough life. Quite probably being from the Narrows if he's a Gotham native, or from some similar place in another city if he's not. In that aspect, I suspect that parts of both his stories are true.
He probably was subjet to domestic violence, or at least was a witness to it (in the Gambol story his father kills his mother before he gives him the scars). And judging from the Rachel story, he may have been somehow betrayed by a woman at some point of his life.
Also, in the TDK script, during the meeting with the mobsters, he implies that his mother was a hooker via the line "Like my mother used to say, if you're good at something, never do it for free." In the actual film, the line is said without the mother part. It's impossible to say how much truth there is in this implication but, IMO, it's not unimaginable and improbable.
Another interesting tidbit is in the early stages of the viral marketing campaign. There was an e-mail from the Joker in which he mentioned a lonely and shy girl named Jessica who was his first kiss and loved his smile. A connection to the Rachel story in which the Wife disapproved how he was always worried and told him he ought to smile more?
So that's what I think about the Joker's background: he comes from a poor family, probably in the Narrows; he may have been a victim of domestic violence, or at least witnessed it; he may have been emotionally hurt by a woman; he was probably driven to crime early-on in his life because of his poor background and because of the fact how versed is he in it; he probably recieved his scars relatively later in life, because he began playing with the Joker gimmick only about a year before TDK and they looked kinda fresh (so definately not a case of child abuse).
That's my 2 cents anyway.