Are Congressional Grants of Immunity, as Currently Implemented, Constitutional?

The House Judiciary Committee wants Monica Goodling to testify at a hearing about the firing of the U.S. Attorneys. Goodling pleads the Fifth. The House Judiciary Committee did what prosecutors often do when they want someone to testify despite the privilege against self-incrimination -- they granted immunity against prosecution based on the testimony. This means Goodling need no longer worry that her testimony will incriminate herself about past crimes, and thus can't claim the Fifth Amendment. (She still has to testify truthfully, or face a perjury prosecution, but the Fifth Amendment doesn't speak to that.)

But is this constitutional? My sister-in-law Hanah Metchis Volokh has a new article arguing that it's not, because such unilateral action by a House Committee violates the separation of powers. (Of course, the article is about immunity grants generally, not the Goodling grant in particular.) Here's an excerpt from the Introduction:

The privilege against self-incrimination requires the government to make a decision: it can either prosecute someone on criminal charges and allow him to remain silent, or it can grant immunity, compel him to testify, and give up the prosecution. In some circumstances, compelled testimony is a good choice -- it can lead to the conviction of a "bigger fish" or allow Congress to gather information that will lead to beneficial legislation or better government oversight. In other cases, the compelled testimony would bring disappointing results and a criminal would nonetheless go free.

The essential separation of powers question is whether Congress has the constitutional authority to make the choice between these competing goals, or whether that choice is committed to the prosecutorial discretion of the executive branch. A federal statute, 18 U.S.C. ยง 6005, allows a single house or committee of Congress to vote to grant a congressional witness immunity from prosecution, thereby requiring the witness to give full and complete testimony at the hearing.

I argue in this Note that the congressional immunity statute is unconstitutional because it violates separation of powers doctrine. The statute unconstitutionally allows a committee of Congress to dictate prosecutorial decisions to the executive branch and to make changes to legal rights and duties without using the legislative process laid out in the Constitution. Whether the power to grant immunity is characterized as executive or legislative, it may not be exercised by a congressional committee. The decision whether or not to prosecute and the decision of which evidence should be used in pursuing a conviction are matters within executive branch control. Congress can make binding decisions in these areas only by statute.