Extraordinary rendition does indeed violate US law, as that is the whole purpose of extraordinary rendition. It is the practice of extraditing someone without the due process of law and that is the source of the controversy regarding the practice. So by its very nature it violates US law. Anyone saying otherwise is simply trying to fool you.
No, extraordinary renditions do not violate US law, because the United States does not extend to other people the same rights that it extends to its citizens. The US has always gone beyond the rights extended in its constitution when it comes to matters of national security, and extraordinary renditions are no different.