And the fact remains no slaves were freed by the EP. It was a (smart) political move to change the issue of the war to slavery and keep the British and French out. If the Brits/French had gotten involved the North would have been toast.
Of course that correct, but you skirt the question. Was it constitutional? I say yes it was. A bit fascist maybe, but not unconstitutional. As has been pointed out, it was aimed at a small parts of a few states (not even whole states) in rebellion. Those states obviously didn't recognize the authority of the constitution, so it was no more unconstitutional than, say, if George Bush used the war powers act to mobilize against Al Quaeda, for example.
Answer the question. Whether the military action against the CSA was legal was another matter, but was the EP illegal?