Sure. That's how our system is set up. Congress has the ability to overturn such a decisions through a Constitutional amendment.
Congress is elected by the people and thus the people either through their elected representatives or through their own actions when necessary are the final arbiters over what is and is not constitutional.
They do have that power. They can always change the Constitution. However, the Judicial branch is the final arbiter of the Constitution as it is currently written.
No. The Supreme Court is the final arbiter of those things that are or are not constitutional. And, as Article III pretty clearly explains, it is a very limited jurisdiction. But Congress has expanded the jurisdiction of the lower courts and judicial activists on the Supreme Court have expanded what they think are Article III questions.
Lower courts only have what jurisdiction Congress gives them. The Supreme Court only has what jurisdiction the Constitution gives it. And there is even a remedy in Article III for those judges (Supreme or otherwise) who step outside of that jurisdiction.) Not that it will ever happen, but thems da facts.