Not discouraging new answers, but: You might find this answer from u/the_alaskan to be useful to the question of how many Southern citizens owned slaves. I also wrote about it here, in the context of what we mean when we say someone owned property in the antebellum US -- you can pretty easily find stats that e.g. only five percent of all Americans owned slaves, which is true but also misleading for a variety of reasons.

I wrote about the question of "how did people feel about the war" in the context of Missouri, a border state. You can find that old answer here.

Since those are all old threads and locked, feel free to ask follow up questions here.

The poor whites in the South were none too happy about fighting in a civil war started by the plantation slave owning elite. They considered it a "rich man's war, but a poor man's fight".

A lot of the division had to do with geography. The hilly/mountainous parts of the South without large plantations or slaves didn't care if slavery survived or not. West Virginia broke away from Virginia proper when Virginia voted to secede. They were more connected with the economy of the Ohio river valley and therefore with the North.

The pre civil war legislative apportionment in many southern states was rigged to insure that the lowland areas with the large plantations would always dominate state govts. Slaves were counted toward population even though they couldn't vote. So even before the war the upcountry farmers had legitimate grievances against the slave owning elite.

The Confederate govt made things worse when they enacted conscription laws. Initially owners of 20 or more slaves were exempt. This provision was later repealed but rich men could still buy exemptions that poor families could never afford. Additionally since the poor families had no slaves they needed all the male family members to work their farms or they might starve. The high desertion rate from the Southern military was partially caused by men trying to escape so they could tend to their land.

Because the North blockaded the South early in the war the Southern army was always short on supplies. Requisition squads would raid the upcountry farms, forcing the poor farmers to contribute to the war effort. Since many upcountry farmers were known to support the Union, and even had family members that went North to join the Union army, the inhabitants were often brutalized by the Confederate army.

During reconstruction the only white support many Southern Republican governments got was from the upcountry areas that had supported the Union during the war.

I got all this from Reconstruction by Eric Foner.