The secrecy is all in your imagination. Obviously the cabinet knew about it. Who else needed to know?
Nonsense, from here
"...Within a few days,...Seward's handpicked army and naval officers had followed Fox to New York with $10,000 that Seward had procured from the secret service budget....the embattled Secretary of State had passed far beyond the jurisdiction of his department, and he was acting so much in isolation that no one in the administration even knew of the expedition but Lincoln."
You said: "Obviously the cabinet knew about it.
Well, obviously they did not.
Are you ignorant or just making up stuff as you go along?