Well, I feel I pretty much bagged civil war as an idea by focusing on the real civil war, but I guess you can do this, to.
I feel using the Battle of Fort Sumter isn't the best idea. A battle where no one actually died means there's no risk whatsoever in the story unless the protagonist fucks up the timeline by firing a mortar shell at 90 degrees. Plus, the battle was pretty much impossible for it to go any other way, so there's not a lot of important effect. And even if it did change up a bit, it was relatively unimportant, except as a symbol.
So basically, pick another battle. I'd make a suggestion, but US civil war isn't my thing. Best of luck, anyway, hopefully you find what you're looking for.