The North cannot properly be accused of treason in that it was the Southern states that seceded and made resort to armed force against federal troops and installations. A seasoned lawyer, Lincoln knew that letting the South strike first put them in the wrong under the constitution and legitimated the North's position.
Churchill was correct in his assessment of Southern slavery. It created a society that was dominated by an arrogant baronial class of large slave holders determined to cement themselves and their families in power and to be secure against any interference from the North or anyone else. Against good sense, they deluded themselves into thinking that they could secede and then win the ensuing military conflict in spite of the South's inferiority in population, finance, and industry.
Alternatively, the South could have accepted Lincoln's election but created a Southern confederacy within the federal system, engaged with the North, and delayed a reckoning over slavery as she built up her military potential. We may add to the list of slavery's ill effects that it made the South's leaders arrogant and stupid.
Of course, no later than the beginning of the new century, with the boll weevil and the exhaustion of Southern soils, cotton cultivation and even a reformed system of slavery with gradual emancipation would have collapsed into some form of the sharecropping system that defined the South's agricultural economy after the Civil War.
In any event, the North's appetite for freedom and equality for Blacks would have quickly diminished as free Blacks made their way North -- which is more or less what happened after the Civil War. Even without that tragic and unnecessary conflict, slavery was doomed to fail, and Jim Crow and segregation were destined to rise in its aftermath.
The South responded to Lincoln's skillful use of the U.S. Navy to provoke the Gulf of Tonkin Incident - err, I mean the Fort Sumter Incident. Let's set that aside.
When you write about the “North's position” - what position was that?
The passage “levying war against them” certainly gave Lincoln the language he needed. It would have been such a perplexing situation if Jefferson Davis had been able to forbit North Carolina from firing on Fort Sumter.