“The feds replied in kind”
“In kind” is precisely not how they replied. Preserving the union, or more accurately conquering the South and forcing it back into the union, has nothing to do with an in kind response to Sumter. I have no idea how history was ever so fooled. Except that everyone now thinks slavery is bad, and therefore the South must be to blame.
“The South did attack first”
Your mind operates in a manner foreign to me. This is true, so far as it goes. But you say it as if it means the South invaded the North and tried to turn it into a client state perpetually under the thumb of Richmond. Certainly this is what Lincoln, too, implies, otherwise the Gettysburg Address makes no sense.
However, the attack was a casualty free shelling of a fort, which amounts to theft. The war that followed seemed to use it as a pretext, but of course repatriation of rebels has nothing to do with stealing federal property. The war the North was justified in continuing after Sumter, like I said, was a war to retake lost property, be recompensed for its trouble, and to some extent punishing the other side. They weren’t justified in conquest. Or if they were, not by Sumter.
The war ended over a 150 years ago. The Union was preserved. So do your "what if's" should the South had prevailed.
Best thing to do is get over it.