It's clear that the total military, economic, political, and human destruction caused by the War was many times higher than the cost of the slaves.
It's also clear that residents of Northern states would never agree to mass re-settlement of former slaves in their own states.
The War might have still been fought, but the North could never claim a Moral or Constitutional victory after rejecting that offer.
“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.