All true, but slavery had to go.
Slavery was going to go inevitably anyways. Eventually plantation slavery would have become more costly than agricultural machinery and slavery would have then ended for reasons both economic and humanitarian.
But slavery wasn't the reason why the Union fought the civil war. The reason the Union fought the Civil war was because an Independent South posed a serious economic threat to those North Eastern industrialists which later came to be known as "Robber Barons." (And who's ideological descendants are still running things from New York/Washington D.C. today.)
The Civil War was fought for economic reasons, and "Slavery" was just the propaganda to rouse public ire. Lincoln was sending assurances that he would keep slavery for the first 18 months of the war. It wasn't until it was made clear to him that the South absolutely wanted independence that he threatened them with the Emancipation Proclamation.
Had the South given in prior to 1863, Lincoln would have restored everything to what it was prior to the war, including slavery. How do we know this? He said so.