Since Justice Campbell dealt with only Seward and never spoke directly with Lincoln how do you know that Seward wasn't lying or, more likely, telling Justice Campbell what he believed Lincoln should do rather than what Lincoln planned to do?
As a representative of the Federal Government, of Lincoln and the Executive branch of government, the responsibility stills falls to Lincoln. If you can't delegate responsibility, then who needs subordinates?
But then again, what you impute is that not one, but two US Supreme Court justices are liars, as well as the 3 peace commissioners, one of whom was also a judge.