“The South should have offered to sell 100% of the slaves to the North....”
Wrong, because the secessionists made it abundantly clear they had every intention of using the Confederate States of America to preserve and extend their slavery into new lands, including their conquest of Cuba, Mexico, various other Latin American states, and whatever part of the United States territories where they could gain some form of control.
I have not studied the Civil War - or the Old South - in sufficient depth to debate your argument.
I do know the Old South was deeply concerned about being outvoted in the US Congress, and being outvoted in presidential elections, and those issues were a major factor in Secession and their decision to fight.
Since 90% of the Confederate soldiers did not own slaves, I have always been skeptical that slavery was the central issue for those who actually risked their lives for the Old South.
In fact, since 90% of the men fighting for the Confederacy were not well educated or highly skilled, it appears to me that those soldiers were in direct competition with slaves for many jobs, and that slavery would have crushed the wage scale for all low skill jobs in the South, the same way massive legal immigration in 2015 has crushed the wage scale for all low skill jobs in America.
Your claim that the Confederate states planned to defeat the much larger and more industrialized North in a war, and then planned to conquer and enslave Cuba and Mexico, sounds ridiculous.
If you are claiming that some people in the Old South believed that might happen, or wanted that to happen, well, OK, I believe that.
But if you are claiming that large numbers of Confederate soldiers were fighting for that goal, or even believed that such a goal was possible, well, that sounds ridiculous.