In regards to point I., I don't think the opinion of a group of Southern aristorcrats elected to the Seccession Convention is necessarily representative of the average farmer. Just because they said that slavery was a primary reason, doesn't mean that the majority of the people thought that slavery was the primary reason.
While there were probably as many reasons to fight as there were men fighting, let me point out a few things.
1) Texas, Virginia, and Tennessee held referendums on their Ordinances of Secession. Two-thirds of the voters in Tennessee, three-quarters of the voters in the other two states voted in favor of the Ordinances, so clearly a lot of the common folk thought they had a stake in slavery, too.
2) Prominent Southern politicians, such as Alexander Stephens in his Cornerstone Speech, deliberately attempted to rally the common man, using the preservation of slavery as one of the rally cries.
Why would the people of the South be fighting for slavery, if the majority don't own slaves? Why would they fight so passionately for a cause in which they had no stake in?
In 1860, about 10% of southerners (one third of all families) owned slaves. In 1960, about 5% of Americans owned stock – that doesn’t mean most Americans had no stake in the capitalist economic system.
Slavery was the backbone of the southern economy and its uncompensated loss is one of the reasons the south was so economically troubled in the years after the war. Besides, a man who did not own slaves could aspire to better himself by eventually purchasing one. And even those too poor to ever dream of that would often support slavery, because it meant they were at least higher status than someone and/or they feared what a slave rebellion could do.
Not that everybody in the Confederacy did. As you say, some were fighting to defend their land. Some did fight for the concept of State’s Rights, though the Fugitive Slave law had shown that their political leaders were willing to discard State’s Rights so long as it helped preserve slavery. Others, like their Union contemporaries, signed up because of a sense of adventure or because they didn’t want to be shamed by staying home.
And many didn’t want to fight for the Confederacy. West Virginia sure didn’t. Neither did eastern Tennessee, northern Georgia, southwestern Virginia, and much of Texas. The Union army was made up of volunteers, the Confederacy had to institute the draft. 10% of the adult male white population from the southern states fought for the Union, while other did their best to avoid military service.