Well, if you actually read the Emancipation Proclamation, it says right there: “ by virtue of the power in me vested as Commander-in-Chief, of the Army and Navy of the United States in time of actual armed rebellion against the authority and government of the United States, and as a fit and necessary war measure for suppressing said rebellion”
An Obamaesque reading of the Constitution. The point that Lincoln was a dictator isn’t original with “Lost Causers” (which epithet shows you really don’t want to think). It is a point that was made during Lincoln’s administration, after, and later by NEW DEAL (i.e. liberal) historians such as Clinton Rossiter.
And, just to help you out, nothing I wrote was a complaint that Lincoln didn’t “free” more slaves. It was an accurate observation that where he had the power to actually set slaves free, he didn’t (with the exception of DC).Lincoln opposed slavery, but it was a ways down his list of priorities. He also wanted to send blacks back to Africa (the DC emancipation had money in it allocated for that, and sending black back to Africa was always something Lincoln favored.
Which brings us back to the original question of what power he had to set the slaves free in areas not in rebellion. Lincoln's view can be seen in the lines you quoted above about the Corwin Amendment: "“I understand a proposed amendment to the Constitution has passed Congress, to the effect that the Federal Government shall never interfere with the domestic institutions of the States, including that of persons held to service. Holding such a provision to now be implied constitutional law, I have no objection to its being made express and irrevocable.”
Lincoln didn't think he had the constitutional authority to end slavery in areas not in rebellion. That's why he pushed for the 13th Amendment.