None of that negates the South voted to leave the Union over slavery! Who historiically started the Civil War? And said war was not against Lincoln's wishes?
First statement in your post:
Why should not the South have been allowed to leave the "union."
The question was not over "slavery" but over authority.
Prior to that terrible conflict, the states held vast authority over nearly every aspect of the citizenship. There was no federal bureaucracy in which has held as superior to the "states' rights."
The conflict was first and primarily about authority and in particular at the local level and also at the national level.
The North wanting a tax structure that would cripple the southern economy.
The South wanting a tax structure that would cripple the northern industries.
It came down to money and authority. Same thing is happening in our modern times.
First question in your post:
It was the North that started the conflict by refusing to leave Sumter and Lincoln restocking it. And then historically, the North consistently invaded the South, not the other way around.
With the exception of Lee taking the battle to the North, and had he been in better health had won at Gettysburg, the Yankee's who were already suing for peace on multiple fronts would have capitulated the cause.
Second question in your post:
Historically, Lincoln did not desire the war, however when the issue was thrust into his administration, did as he was expected.
Had there been any other alternative in which he considered viable short of allowing the south to depart, then he would have taken it.