Says the Chief Justice in response to Lincoln's total usurpation of power in Ex Parte Merryman:
"With such provisions in the Constitution, expressed in language too clear to be misunderstood by any one, I can see no ground whatever for supposing that the President, in any emergency or in any state of things, can authorize the suspension of the privilege of the writ of habeas corpus, or arrest a citizen, except in aid of the judicial power. He certainly does not faithfully execute the laws if he takes upon himself legislative power by suspending the writ of habeas corpus — and the judicial power, also, by arresting and imprisoning a person without due process of law. Nor can any argument be drawn from the nature of sovereignty, or the necessities of government for self—defense, in times of tumult and danger. The Government of the United States is one of delegated and limited powers. It derives it existence and authority altogether from the Constitution, and neither of its branches — executive, legislative or judicial — can exercise any of the powers of government beyond those specified and granted."
It doesn't really matter what Congress thinks, because Congress is not the judiciary. You seem to have trouble with separation of powers.
Neither are you. Since the Supreme Court has not ruled on the question then you can hardly say Lincoln's actions were unconstitutional.