1) Abe Lincoln won the popular vote amongst the 4 candidates.
Lincoln: 1,865,908
Douglas: 1,380,202
Breckenridge: 848,019
Bell: 590,901
2) It does have its roots in slavery. You may have heard of the this 3/5 compromise.
3) Doesn't need an amendment. https://en.wikipedia.org/wiki/National_Popular_Vote_Interstate_Compact