An old post of mine (sorry for any links that may no longer work) at
https://www.alternatehistory.com/forum/threads/worst-union-president.438048/#post-16615102 on Seward, who would certainly be the most plausible Republlcan president if Lincoln were not available:
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A case can be made that Seward, one of the most able men considered for the Republican nomination, might actually be the one most disastrous for the Union. I used to think that a President Seward might unintentionally allow peaceful secession. By "unintentionally" I mean that of course Seward wanted secession to fail--but he had an unrealistic idea that if the North just avoided conflict by abandoning Sumter and possibly Pickens as well [1], not only could the Upper South be held but a Unionist reaction would develop in the Lower South, leading to reunion. (He also thought that a war scare with Spain--I am not convinced he wanted an actual war--could bring about North-South reunion in the interests of "patriotism." See
https://groups.google.com/d/msg/soc.history.what-if/HfDoPtlOem0/gbVpY6q3OzsJ for a discussion of this.) By the time he realized that voluntary reconstruction was a pipe dream, the independence of the Lower South might be so established that he could do little about it.
I am now inclined to think, however, that Seward was such a staunch Unionist that once it was clear to him that his strategy for bringing about voluntary reunion by abandoning the forts had failed, he would have to resort to some sort of "coercion"--even leaving aside his own strong nationalism and expansionism, it would be politically disastrous for the Republicans to be known as the party that accepted disunion. The most likely method, once the forts were gone, would be an attempt to collect the revenues offshore.
[1] There is no actual proof that Seward advocated it, but most historians believe that Scott's sudden recommendation that Pickens as well as Sumter be abandoned--and on openly political (basically "Upper South Unionists insist on it") rather than military grounds--must have come from Seward. E.g., William Cooper in his We Have This War Upon Us: The Onset of the Civil War, November 1860-April 1861:
"No evidence makes Seward's involvement indisputably clear, yet little doubt can exist. He had had Scott's confidence since the winter, before Buchanan left office. He and the general had become closer, though Gideon Welles surely exaggerated in calling Scott no more than Seward's pawn. The secretary of state and the general had given Lincoln identical advice on Fort Sumter since the outset of the crisis. But Scott had never before mentioned politics in suggestions he had given his commander in chief. Seward knew firsthand, however, that his Conservative Unionists wanted federal authority gone from Pickens as well as Sumter. If Seward and Scott had hoped with this double-barreled counsel to bring the president closer to them, they grossly miscalculated..."
https://books.google.com/books?id=LT5_dhd8JNMC&pg=PA249