“There was no constitutional authority to outlawing slavery in any state’s borders”
There was no explicit authority, but of course the Constitution doesn’t specify everything a commander in chief can do, nor what everything that’s different about a time of emergency as opposed to peacetime. I’m not much for implied powers, and contra-Marshall would demand we give the benefit of the doubt to not doing what the Constitution doesn’t say you can. But to say that there is no extra authority in times of war just can’t be.
We can argue over the necessity and morality of the EP, but not dismiss its constitutionality out of hand. It makes a pretty good case for itself in its own text.
“the EP was a wartime measure aimed at areas in ‘rebellion’”
Yes.
“and they feared the measure would expire when there was peace”
Yes.
“they didn’t nevermind this”
Neither did I.
“but openly expressed how seriously they believed this would happen and urged the 13th, especially after his assassination.”
Okay, so there weren’t mere doubts, these doubts were Hugh and Series, therefore the EP wasn’t worth the paper it was written on. Did it ever occur to you that it could have stood on its own merits, no matter what abolitionists thought? For all your talk about the principle of secession being bigger than and not living or dying with the Confederacy, I wouldn’t think you of all people would wrap the EPs constitutionality up in what partisans at the time thought might happen. The EP’s legality supercedes any and all considerations of practicality.