I know in the context of the film, we are told if Dent messes up everyone will come after him and it's all on him but I'm not sure if legally that makes sense. They wouldn't just let the criminal go if the guy who arrested them becomes a murderer. TDKR doesn't spend any time explaining what really happened after Dent died. Who prosecuted the criminals? Did his death inspire more noble politicians? I know it's a super hero movie, so it's not a huge issue. And about corruption, Gordon agreed to cover up Dent's crimes - that's corruption. That's not his job. He never seemed happy about it, it was Batman's choice. It takes it's toll on Gordon as we see the truth is eating him away and his wife leaving him is more proof that he didn't get away scott free.