I agree with everything you wrote. To add just a bit to it,

the southern objective was to expand slavery into areas where it was not allowed. The single reason for this was that slavery was extremely profitable. The arrival of the industrial age with Whitney's improved cotton gin and the steam looms in huge factories drove that profitability. There was a vast increase in the value of slavery between 1787 when it was economically marginal and 1860 when it was awesomely profitable.

When the southern aristocracy concluded that they would not be effective in expanding slavery as a part of the Union, they decided to leave the Union to achieve their objectives in the West, North, Mexico, and even (possibly) Cuba. That was all going well with legislatures and conventions voting to stay (Virginia did) or leave (e.g., South Carolina). Those that left told the Union that its possessions (e.g., forts, post offices) had been confiscated and ordered them vacated.

It was going smooth as silk.

Lincoln said "In your hands, my dissatisfied countrymen, lies the momentous issue of civil war. We shall not assail you." Whether he felt that force was justified to compel the seceding states return to the Union or not, we shall never know. He was being very careful with an explosive political situation.

Then the hotheads in Charleston decided that the Union garrison in Fort Sumter was taking too long to leave and, despite the fact that Major Anderson had made clear that his supplies were running out, it was time to start shooting.

The rest, as they say ...