Lincoln was in Springfield at the time and had not appointed Seward or anyone else to his cabinet. What evidence do you have to support your theory that Lincoln was pulling strings from hundreds of miles away, or that Seward was willing to allow Lincoln to pull the strings
The fact that Lincoln met with Thurlow Weed in Springfield just before Christmas in 1860. Weed was a newspaper man from New York who had worked on the political campaign of Lincoln and was an associate of Seward, the New York senator. Seward, who it was becoming increasingly clear that Lincoln would make SoS, had Weed go to Springfield to discuss the secession crisis and make plans on the political spoils appointments of the new administration.
At the conclusion of the meeting, Lincoln instructed Weed specifically on several compromise proposals to put before the Committee of Thirteen in the Senate and sent Weed to communicate them to Seward with instructions to propose them before the committee. Lincoln simultaneously informed Senate allies Hannibal Hamlin and Lyman Trumbull at the same time of the activities of Seward and to expect them before the committee in late December. Seward arrived in Washington after meeting with Weed to recieve the message, proposed the Corwin amendment before the committee, and a day later wrote Lincoln in Springfield informing the president elect that (1) his message had been recieved, (2) his ideas had been drafted into three proposals and introduced before the committee as he requested, and (3) one of them was the Corwin amendment protecting slavery.
Lincoln's meeting with Weed was publicly known, but his involvement in the amendment and other compromise efforts were kept secret early on. For that reason, Lincoln specifically asked Trumbull and the others in letters to keep quiet about his involvement in the amendment, partly because he was simultaneously avoiding efforts by the Buchanan administration to get him to support competing proposals.