"But think of this way: If one subtracts the issue of slavery as the cause for the "irrepressible conflict," what ,then, forced the country to enter into its bloodiest conflict?"
The correct question would be "what anticipated consequence motivated Lincoln to send the Navy to Charleston in April of 1861?"
Good one, PR, but the question is nearly moot in that South Carolina seceded in January, 1861. But yes, Lincoln was a clever man and he knew well what would happen if he tried to resupply Fort Sumter. But the spark for war certainly would have occurred somehow; if not in Charleston, then somewhere else.
Thanks for your very cogent post!