I have a theory. Kanjuro developed a something like a split personality. One that is loyal to Orochi, and one that is loyal to Oden and the Scabbards. The personality loyal to Orochi isn't aware of this, but the one loyal to Oden is aware. Because of that when Kanjuro wanted to write a letter to warn Orochi, he subconsciously wrote that everything fine, and he can let his guard down. Also, the guards in the tunnel knew Kanjuro is the traitor, but he still attacked them, telling himself they weren't aware. I could be wrong, but this is what it looks to me.