Lincoln knew exactly what would happen if he attempted to blockade the harbor, because 5 states had already seceded when Buchanan tried to do that. The war was deliberate. No one ever thought secession was illegal, and granting the Federal government the power to use military force against a state was deliberately and soundly rejected at the Constitutional Convention. Everyone also knew what the Republican platforms were, implementing the old Whig 'American System', a series of government projects that would only benefit the northern states and radically change the tax laws and how government was funded, with massive railroad subsidies, the Moril tariffs and homestead acts to go along with the railroad subsidies.
So yes, he could have mediated if he so chose to. The South's 10% tariffs and the plan to ship cotton directly to Europe and bypass the northern shipping and financial monopoly tipped the balance to go to war in Lincoln's favor, barely; New York City almost seceded as well, and Pennsylvania almost went, too. He openly rejected any negotiations or legals means to resolve his quandary, he needed other baseless pretexts for window dressing. One scheme was to use the Federal naval ports to tax southern exports. If they resisted this scheme, he would use that as a pretext.
Nobody expects some politician to come out and say the real reasons they are doing something; because Lincoln didn't say 'I plan to hand my cronies all kinds of subsidies and pass a lot of the tax burden off on southerners and working people' doesn't mean they didn't do exactly that, right off the starting line. We have the record of their intentions in the congressional record, in the bills they passed first before they ever got around to slavery; his Emancipation Proclaimation nothing but a ploy to keep England out of the war via a fake 'morality' gimmick.
"But what am I to do in the meantime with those men at Montgomery [meaning the Confederate constitutional convention]? Am I to let them go on... [a]nd open Charleston, etc., as ports of entry, with their ten-percent tariff. What, then, would become of my tariff?" ~ Lincoln to Colonel John B. Baldwin, deputized by the Virginian Commissioners to determine whether Lincoln would use force, April 4, 1861.
"You and I both anticipated that the cause of the country would be advanced by making the attempt to provision Fort Sumter, even if it should fail ; and it is no small consolation now to feel that our anticipation is justified by the result. "
Abraham Lincoln, in a letter to Gustavus Fox, May 1, 1861
"Union means so many millions a year lost to the South; secession means the loss of the same millions to the North. The love of money is the root of this as of many other evils....The quarrel between the North and South is, as it stands, solely a fiscal quarrel".... Charles Dickens in a London periodical in December 1861
"The contest is really for empire on the side of the North and for independence on that of the South....". ..... London Times of 7 Nov 1861
"Slavery is not the cause of the rebellion ....Slavery is the pretext on which the leaders of the rebellion rely, 'to fire the Southern Heart' and through which the greatest degree of unanimity can be produced....Mr. Calhoun, after finding that the South could not be brought into sufficient unanimity by a clamor about the tariff, selected slavery as the better subject for agitation"..... North American Review (Boston October 1862)
"They [the South] know that it is their import trade that draws from the people's pockets sixty or seventy millions of dollars per annum, in the shape of duties, to be expended mainly in the North, and in the protection and encouragement of Northern interests....These are the reasons why these people [the North] do not wish the South to secede from the Union." ..... New Orleans Daily Crescent 21 January 1861
"In one single blow our foreign commerce must be reduced to less than one-half what it now is. Our coastwise trade would pass into other hands. One-half of our shipping would lie idle at our wharves. We should lose our trade with the South, with all of its immense profits. Our manufactories would be in utter ruins. Let the South adopt the free-trade system, or that of a tariff for revenue, and these results would likely follow." .... Chicago Daily Times December 1860
"At once shut down every Southern port, destroy its commerce and bring utter ruin on the Confederate States." ..... NY Times 22 March 1861
"the mask has been thrown off and it is apparent that the people of the principal seceding states are now for commercial independence. They dream that the centres of traffic can be changed from Northern to Southern ports....by a revenue system verging on free trade...." .... Boston Transcript 18 March 1861
"The affair at Fort Sumter, it seems to us, has been planned as a means by which the war feeling at the North should be intensified, and the administration thus receive popular support for its policy.... If the armament which lay outside the harbor, while the fort was being battered to pieces [the US ship The Harriet Lane, and seven other reinforcement ships], had been designed for the relief of Major Anderson, it certainly would have made a show of fulfilling its mission. But it seems plain to us that no such design was had. The administration, virtually, to use a homely illustration, stood at Sumter like a boy with a chip on his shoulder, daring his antagonist to knock it off. The Carolinians have knocked off the chip. War is inaugurated, and the design of the administration accomplished." ~ The Buffalo Daily Courier, April 16, 1861.
"We have no doubt, and all the circumstances prove, that it was a cunningly devised scheme, contrived with all due attention to scenic display and intended to arouse, and, if possible, exasperate the northern people against the South.... We venture to say a more gigantic conspiracy against the principles of human liberty and freedom has never been concocted. Who but a fiend could have thought of sacrificing the gallant Major Anderson and his little band in order to carry out a political game? Yet there he was compelled to stand for thirty-six hours amid a torrent of fire and shell, while the fleet sent to assist him, coolly looked at his flag of distress and moved not to his assistance! Why did they not? Perhaps the archives in Washington will yet tell the tale of this strange proceeding.... Pause then, and consider before you endorse these mad men who are now, under pretense of preserving the Union, doing the very thing that must forever divide it." ~ The New York Evening Day-Book, April 17, 1861.
.... and dozens more, both pro and con papers all over the U.S.. Everybody at the time knew what the war was over: northern welfare programs paid for at the expense of the South.
So yes, real history does make the ignorant look like morons, especially those who keep trying to bullshit everybody that they would have been big anti-slavery warriors fighting to free back people n stuff and should get some sort of imaginary moral authority on modern issues, when we all know they wouldn't have been any different than anybody else back then.
And, we know what Lincoln's plans for those 'freed' slaves was, too, since he already implemented that plan under the military governors in the states the north had occupied during the war as well. It was keeping them on the plantations, without the right to leave without permission from the owners, but since they were 'free' they had to be paid a wage: $3 a month, set by the government.
