Ryujin
Legend
To address this specific part separately, I have to point out that you appear to be assuming that the Joker was never in prison before becoming the Joker. If the Joker was a regular but eccentric criminal before whatever caused him to become the Joker happened, then being affected by or caring about prison culture makes some sense.
Also, I have a personal theory that the damaged tattoo is not something the Joker did, but is something that others did to him at some point in his past. That tattoo's gentle cursive quality doesn't fit in with the rest of the more jagged and angular tattoos on the joker.
Yes, pretty much. I'm also assuming that The Joker would divorce himself from his life previous to his "rebirth" as much as possible. That was another thing that appealed to me about Ledger's performance; the constant retelling and reinvention of his scar's origin, as he told it to different characters through the movie.