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.
I will not argue with your post.
Recently I posted about obstruction of governance or conspiracy to obstruct governance.
That is precisely the course of the liberals at present. The Judges ruling about the executive order in plain dismissal of the law passed by the congress is obstruction of governance. There was no regard for the law. Law was made up solely to obstruct governance.
America is presently in the throes of a full blown constitutional crisis deliberately brought by the liberal fanatics to overthrow the Trump government by obstruction of governance.
There is but one recourse. That is to come down swiftly and aggressively on the obstructors and remove them from society into a place where they can not act at all.
If it is ok to disregard the laws protecting America, it is ok to disregard the law protecting the obstructors. If there is no law, then there is no law.
For these reasons, I consider arguments regarding congressional oversight trivial at present. The problem lies with the Judicial Branch and can not be remedied with out very harsh action against the thousands of lawyers and judges all across America.