What is "this" proof that you speak of?
No one can argue for -or- against "I read somewhere..." "I remember seeing/hearing on a panel..." "I'm pretty sure so-and-so said..."
Hearsay is unprovable by definition. I read a lot of rumours that Ledger was originally supposed to be in it, but I didn't believe any of it because none of it came from a legit or direct source. Again, if anyone can link me to any video, direct primary source quote,...anything legit that says Ledger was supposed to be in Batman 3, please do so.
It is very easy to find Nolan's denial that any footage of Ledger was going to be in it, why is it so difficult to find anything that says the opposite? Is it because it doesn't exist?