I understand it was the south that fired on Ft. Sumter, but if the North (and Lincoln) wanted peace above all else then Ft. Sumter could have been abandoned by the federal government and Lincoln could have negotiated with the south.

My specific questions are as follows:

-Did the southern states have a constitutional right to secede (or is this debatable)?

-Was Lincoln unwilling to let the Union fall apart? Or was this about freeing the slaves? Or both and to what degree of importance each had? (I read his goal was to end slavery and basically saving the union would naturally happen as a consequence but I am doing a lot of reading in between the lines too)

-How did Lincoln politically/publicly justify the scale of the war/defeating the south to the public?