Re: Joker Knew of Bane's Existence.
Crane knew about Ra's Al Ghul because he was working for him.
There is no indication whatsoever or any plausible way for The Joker to know anything about Bane.
He was referring to the truth about Harvey Dent being exposed, but I don't think he expected it to take eight years, or for Batman and Gordon to cover it up, especially since he referred to Batman as incorruptible.
He's a silent guardian...a watchful protector...a Dark Schloss...
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