Oh Lincoln was very aware of things. He and Seward were joined at the hip, so to speak. I'm not sure they believed the Confederate States would put up such a long fight though.
This letter predates the facts by a large amount of time. There is no way Lincoln did not understand very well what was going on.
The letter also predates Fort Sumter, so how it relates to Seward 'talking to the SC gov. on Sumpter, behind Lincoln's back is not visible.
Anyones, even Light reading of that period will see how Seward was trying to "Run" the admistration himself, and trying to relegate Lincoln to the backgroud. Since Lincoln had little experience in this type of job, Seward figured he would be able to Lead the Government.
Gideon Welles understood this and noted it in his 3 vol.Diary, as did other Cabinet officials.
Lincoln "understood" what Seward was trying to do, and kept him on a very short leash, notable so, after he discovered Seward working behind his back on the Forts Sumter and Pickens issues.
Even your letter shows Sewards ploy to insert himself between Lincoln and the rest of the Cabinet.
A reading of Doris Goodwin's "Team of Rival" clearly shows all this.
Would have replied earlier, but....yesterday wasn't a great day in my recovery from surgery.
And Now, back to my bowl of Cheese, Garlic Grits, listening to the 2nd South CaroliniaString Band, and reading Machiavelli.
Chow!