A quandary people that support Lincoln find themselves in: If Lincoln could not free the slaves of the northern States by degree then why did he believe he could do so to the southern States if they were still in the union?
Your knowledge of events is lacking.
Lincoln did not go into office hoping to free slaves (and by the way at that point in time, all slave states were considered 'Southern -- MO. KY, MD, & DL)
He did not even think it was possible to end slavery other than over decades, via compensated emancipapion.
Everyone understood that it would would have taken a constitutional amendment ratified by 3/4 of the states to end it and in 1860, that was beyond the realm of possibility when 15 of the 37 states had slavery. (Even today with 50 states, if 15 favored slavery, you could not pass that amendment.
Lincoln's only campaign promise in 1860 was to prevent the further spread of slavery into the Western territories. He stated in his first inaugural address that he had no intention to change slavery where it currently existed, but he would use his power to prevent it's expansion to future states. His belief, right or wrong, was that if the institution were isolated where it currently existed, it would eventually die on its own weight.
Then came the War, which changed everything, which war always does. You must understand, as Lincoln did, that the Emancipation Proclamation did not end slavery, even in the "South". It was strictly an executive order issued by the Commander in Chief at a time of war to the military to confiscated the 'property' of hostile opponents to be disposed with as the executive decided. His decision was freedom for those individuals.
He had no authority to enforce that order in states that were not at war with the United States (KY, MO, MD, and DL) It only applied to States or parts of states that were still in rebellion. And the order did not end the legality of slavery in those areas. For all intents and purposes, slavery was still legal there, just not for the individual slaves that had been 'confiscated and freed by ececutive order.'
It took the 13th amendment to the Constitution to legally end slavery in the United States, and Lincoln supported that.
When considered, the South itself caused the early end to slavery in the United States by resorting to rebellion and war. Without that rebellion, slavery in the United States would have continued long past 1865.