I think there is a limit to that. I am wondering how much sarcasm there is in that phrase “my friend Feinstein”. I do believe Senators have been convicted of crimes. Feinstein should be no different. Their immunity is limited to slander on the Floor of the Senate, I am pretty sure. Conspiring against the GOvernment of the United States is not gonna be a crime granted immunity by the Constitution.
While I agree with you in principle I think you would find that the courts would rule in the favor of the Senator.
If they can find the right to kill a child in its mother’s womb in the constitution finding the right for a senator to slander a judge in the during the process of the confirmation hearings is like finding rotten apples in a orchard in August.