Voldie is certainly powerful enough because DD managed to get through unharmed.
I think Dumbledore made it through not because he was a powerful wizard, but because he was a respective one. The Centaurs didn't clobber him because he knew how to talk to them, not offend them, and treat them with the respect they rarely receive from humans.
Does it? I can't begin to tell you how many text books I've come across that were wrong at least once.
Well, while I do agree with you in a normal sense (like, not in Rowling's world,) if you watch the interview with her and the writer of the movie, she says that she uses Hermione and Dumbledore to give true background information without sounding boring, because it's believable that they'd know it. Dumbledore, because he just knows just about everything, and you can always assume that anything Hermione says she read in a book.
I take this to mean that if Hermione is quoting something she learned in a book, it can just about always be assumed to be true. I know she read that there were only, what, like, seven animagi in a century and there turned out to be at least four more, but this bit of information is different, I think, than the no apparating or disapparating thing being written in Hogwarts: A History.