The southern states secede but do not form a confederation. Could the Civil War have occurred?

South Carolina attacked Fort Sumter…giving Lincoln an excuse to invade.only…South Carolina.

Suppose the other secessionist states said "You are at war with South Carolina only. Retake the fort by sea if you choose to, but we consider sending invading troops through our sovereign states to get there to be an unwarranted act of aggression.

In that case would the North be the one who decidedly started the Civil War? Or could the war have happened at all?

Lincoln would have eventually have felt he needed to declare war on all the rebellious states. He was not going to accept them seceding and they were not coming back voluntarily.

In the scenario you describe, Lincoln probably would have tried to retake South Carolina first and then used that to put pressure on the other states. Basically make them a carrot or a stick offer - “Come back on your own or we’ll bring you back by force”.

Could the North have retaken Fort Sumter by sea with enough manpower and ships?

Fort Sumter retaken. Problem solved. No war.

Probably. Eventually. The United States had conducted a successful amphibious invasion of Mexico in 1847.

Retaking Fort Sumter would have been the start of the war not the end of it.

The problem wasn’t Fort Sumter. The problem was that South Carolina thought they were an independent country and the federal government thought they weren’t. I mean, lets say the army holds on to the forts and South Carolina, for some reason, is ok with that. So then what happens when Lincoln appoints the Collector of the Port of Charleston or Postmaster of Columbia and the South Carolinians don’t let him take his seat?

Even Castro seems to have learned the lesson of Fort Sumter. Up to that point, Lincoln had been careful not to give a causus belli.

If the southern states had not stood together, they would have been much easier to get “back into the fold”. They would have regained SC rather quickly, and then could deal with each other state one at a time. It wouldn’t take long for those in charge of the remaining southern states to unite, and then things would have gone similarly to real life.

IIRC, many South Carolinians had been pretty much rarin’ to secede way earlier than the other states–they had been wanting to secede for years before 1860–but they didn’t dare actually do it until they were sure that at least a good number of their fellow slave states (say, the “Deep South” states that did in fact secede in 1860-61 before Fort Sumter) would join them. South Carolinians might have been crazy (too small for a republic, too big for an insane asylum and all that), but they weren’t that crazy–they never had any intention of simply trying to go it alone.

Secession may have been piecemeal, but as far as I know it was always with the expectation that some sort of new Southern Confederacy would be organized thereafter, rather than seven or eleven or fifteen or whatever the number of new slave-holding independent republics coming into existence alongside the rump United States.