If a question/answer is demonstrably wrong (proven so by reputable references) then I see no issue with it being outright deleted.
That said, surely it would be better to edit the question or answer to indicate why the original wording was wrong e.g.
"Nagini appears in the first book, the snake he sets free at the zoo was Nagini. J.K Rowling said so herself. I imagine that the snake retreated to You-Know-Who until he was at full power, and then He-Who-Shall-Not-Be-Named made her a Horcrux. Just my theory though."
"Nagini appears in the first book, however the quote listed is a hoax - see above link for details"
If the author disputes the changes, they're welcome to come onto chat/meta and argue why it should be changed back.
As far as the specific questions are concerned, I bow to the superior knowledge of others but I believe that outright deletion is a no-no whereas arranging a downvote storm is perfectly acceptable.