What if he tortured him in one day though? Would the blue flowers work???
Simply put, revealing the Joker's origin is a bad idea. It's just not going to be good for the character. As I've said, part of the reason the character in TDK was so great is because we don't know what made him that way. We only know that he told multiple stories about how he
might have gotten that way, but we have no idea if he's lying, telling a bit of truth in each story, so messed up that he actually believes each story he tells is true, if he scarred himself, if the scars caused him to go crazy or if the scars came after he was crazy, ect. ect.
The not knowing, and guessing at what he was, is part of what makes the character fun.
It's like every great horror movie character. Think of Jason, Freddy, or Michael Myers in their first movies. We were given the bare bones of their back stories, and it worked, because not knowing makes the character more intriguing, and in their cases, more frightening.
So no, revealing what happened to the Joker is a
very, very, VERY bad idea. As I said above, part of what made
The Killing Joke so great is his line at the end
If I'm going to have a past, I prefer it to be multiple choice!
He thinks it was "one bad day" that caused him to be the way he was, but heck, he doesn't even know it. He thinks that's what happened, but he really has no idea.
Revealing the Joker's past ruins part of what made the character great. So stay away from that. However, if you work in the Black Mask betraying him angle, after trying to get the Joker to work with him for his own means, that would work much better. Everything else about your idea works very well.