Not really. When the US Government voted for the Corwin amendment in March of 1861, had the South simply remained in the Union, slavery would have likely been permanent.
Yes. The United States government voted for permanent slavery in March of 1861. President Lincoln supported this amendment.
I do have to disagree with your assertion. The republic could not exist forever with slavery. It was fundamentally incompatible. If it hadn't come to a head in 1861, it would have come to a head 10, 20, or 50 years later. It would have eventually come up, and the same strife would have ensued, in my opinion. Obviously, slavery could exist in a totalitarian society as long as that society lasted, but even then, eventually the government would be overthrown.