Not every slave state chose to secede. The free states were coming into majority power and slavery was near its end. The actual effect of the Emancipation Proclamation of 1863 was that by 1865 there were no slaves anywhere in the US. By Dec of 1865, the states had ratified the 13th Amendment.
Before his assasination in early 1865, Lincoln had personally pressed the House of Representatives to get the 2/3rds majority to approve the 13th amendment. The senate had previously gone for it massively. It took less than a year for the states to forever abolish slavery.
Slavery had remained legal in the US due to the early influence of southern voting power. As soon as they lost power, slavery was abolished. There was no huge appetite for slavery in the north, contrary to your implication. An institution maintained for 300 years by southern pressure ended in the blink of an eye -- just as the south lost voting majority.
Hogwash.
The North salivatingly embraced slavery; and introduced it to the South. The slave industry(of the US) got its start, and built it roots in a welcoming North.
Matching their anxious business desires with capital, the North enjoyed a growing & properous slave trade. The return on Northern investment was substantial, and appreciated.
"Not every slave state chose to secede"
True. Those founders of slavery north of the Mason-Dixon threatened to beat up any that would not stay, and be their friend.
But....we like you now.