Just for the record...... the Executive branch is charged with the duty for enforcing the law
There are adequate laws on the books if actually and rigidly enforced
Well, you make a good point. However, it does seem like a legitimate function for Congress to investigate questionable acts of the Executive Branch (and the Judicial Branch). Self-investigation, especially in politics, is an illusion. Each Branch should check, reject, and nullify the unconstitutional acts of the other with constitutionally-based notice and rationale. The Constitution clearly gives Congress the power to impeach and convict unconstitutional acts committed by officials in the other Branches.
(The Judicial Branch has the constitutional power to nullify unconstitutional laws passed by Congress or unconstitutional acts done by the Executive Branch. Though not explicit in the Constitution, the Executive Branch could, with constitutionally-based notice and rationale, ignore unconstitutional SCOTUS decisions (Lincoln reportedly did that) and not enforce veto overrides that, again, are clearly unconstitutional acts.)
Outside of those functions, however, I’m not sure where the Constitution gives Congress the power outside of constitutional impeachment proceedings to perform judicial/enforcement proceedings like subpoena and testimony under penalty of incarceration.