Is quoting not enough proof that the Op never intended to pass off someone else's work as his own? Please educate me.
It should be enough to not get the user banned.
Personally, I think it is plagiarism, because you really need to give credit for what you are quoting.
No, all you need to do is to not claim that you created the text. Quote tags do that.
We do not and should not have requirements for how the source should be provided and that's a good thing. We don't want to get into elaborate dramas because someone posted a broken/fake link or something.