Slavery goes back to before the US was a country. Abolitionists go back to before the US was a country as well.
Yes, it was common to find early abolitionists. The Quakers and Mennonites in general were very much opposed to slavery on the grounds that it was very un-Christian.
Most northern states passed laws against slavery shortly after the US became a country. In many states, emancipation was gradual. For example, a state might forbid any new slaves from being created, but would continue to allow existing slaves to remain as slaves. Or they might be converted to indentured servants, where theoretically they might be able to buy their freedom some day, though in practice many would never be able to afford it. Some states prohibited the sale of any slave, so if you could no longer afford your slave, you couldn’t sell him. You had to set him free. Most northern states passed laws abolishing slavery somewhere between 1780 and 1810, though since the actual emancipation was often gradual, slavery continued to exist in some form after that, in slowly dwindling numbers.
There was also more opposition to slavery in the south than you might realize. When Georgia was a colony (early 1700s) they actually passed a law abolishing slavery.
That is a very complex question.
This is going to gloss over a lot of things in a huge way, but the North was not as unified against slavery as you have probably been led to believe. The abolitionist movement in the North had been growing all through the 1800s, but even at their strongest, the abolitionists did not have anywhere near enough wealth and power to counter the southern plantation owners. But, the northern industrialists and the southern plantation owners hated each other. The industrialists wanted protective tariffs, so that northern factories could produce goods cheaper than European imports. But if the industrialists got their tariffs, then European countries would counter with tariffs on things like tobacco and cotton, which hurt the plantations. If you don’t have the tariffs, though, then the plantations get to sell more tobacco and cotton, but the northern factories can’t produce goods cheaper than European imports, and the factories suffer. So economically, the two groups were very much opposed to each other.
In the mid 1800s, the industrialists and the abolitionists kinda got the idea that the enemy of my enemy is my friend, so they basically banded together and formed the Republican Party. While the abolitionists in general didn’t give two hoots about the economics of factories, and the industrialists in general didn’t give two hoots about slavery, by banding together, they were able to defeat the South politically.
Since anti-slavery was a core part of the Republican platform, this may give you the idea that the entire North was unified against slavery. It wasn’t. There weren’t a whole lot of pro-slavery attitudes in the North, but there were a lot of folks who were fairly apathetic towards it.
In the South, what you had was a combination of the wealth and political power being mostly in the hands of the plantations, which could not survive economically if they had to free their slaves and start paying them wages. The small southern farmers typically didn’t have slaves. They couldn’t afford them. But slavery wasn’t just about unpaid labor for the plantations. It was also a way for the entire South to keep blacks under control, with no power to vote, and no political power whatsoever. The small southern farmers did not own slaves, but they very much believed that blacks were inferior to whites, and were absolutely certain that blacks would completely destroy the South if they were set free and allowed to make their own decisions. So basically, it was blatant racism, treating blacks as if they were some sort of sub-human and violent animal.
Northern attitudes towards blacks were better, but even then blacks were not considered to be the equal of whites, even by most abolitionists. Lincoln himself thought that blacks were not as intelligent as whites, though his views did evolve a bit over his lifetime. There were some abolitionists back then that thought blacks were equal to whites, but they were a definite minority. Most abolitionists thought that blacks were inferior to whites, but also thought that it was cruel to treat them as slaves.