I think the Southerners had a lot to do with that as well.
No other nation required a war to end slavery.
No other nation has a large section willing to go to war to protect slavery.
The amount of money the union spent on the war could have bought the freedom of every slave.
In retrospect that's probably true. But it's also irrelevant because there was no desire in the South to end slavery, either through compensated means or any other.
Probably true. The amount would have been roughly equivalent to the money the Union spent on the War. Add in the amount spent by the CSA and the damage caused, and it would have been much cheaper to buy the slaves out.
The problem is more than what you say. The North had no desire to tax itself to pay slaveowners compensation. Much as very few Americans are interested in taxing themselves today to pay reparations to the descendants of slaves.