Jump right in, my time is limited.
I will take the time to note that the South didn’t invade the North until over 2 years into the war, hoping they could win a defensive war for what they saw as their territory. They were not trying to control the entire country, which would have made it a true civil war in the traditional sense of the term. They were trying to break away as their own group of states.
The rebels were trying to overthrow the government - even if only within one geographical part of the nation. An internal struggle for control of the government is the definition of civil war.
North and south have often found themselves at odds regarding policies and direction of government. But 70+ years of operating as a nation (leading up to the Civil War) shows pretty clearly that the south wished to dominate the nation. Their central focus - their raison d’être - was an economy built upon the backs of slave labor and the southern states always voted en bloc to defend the Particular Institution.
From the inception of the Constitution itself with it’s veiled references to slavery to the Fugitive Slave Act of 1793, the Missouri Compromise of 1820, “Gag rule” in Congress from 1831 through 1844, the Compromise of 1850, the Kansas-Nebraska Act of 1854, and even the Crittenden Compromise of 1860 - any and every attempt to minimize or curtail slavery was met with a “my way or the highway” response.
So when the slavers saw the writing on the wall, they didn’t seek a coup where the seized control of all the states, but they seek to breakaway through an insurrection. I agree that they had no plan to take over the northeastern states - but they did make a play for every slave state and all of the western states and territories.
And their plan did include incapacitating as much of the US military as they could.