“Lincoln refused to meet with Confederate commissioners who came to Washington to negotiate a peaceful separation in February of 1861.”
But then he had no more authority to negotiate a peaceful separation than the Obama administration has to “Peacefully grant the State of...to withdraw from the United States of America and create its own NEW government” in accord with the various petitions we read about.
“He did not seek a constitutionally required declaration of war...”
Why would a declaration of war be constitutionally required? If they were not acknowledging the right to secede, it would be a matter of an internal insurrection, not an external threat. “The Conch Republic is a micronation declared as a tongue-in-cheek secession of the city of Key West, Florida from the United States on April 23, 1982.” If somebody really wanted to make an issue of it, would Congress have to declare war against the Conch Republic just because they say they are a nation, or could Florida just send in the cops/national guard?
You are correct - no declaration of war was required. Lincoln went to great pains to frame the “Unpleasantness” as a rebellion, thereby, among other things, obviating the need to declare war.