As for Akiha, I don't think there was anything "wrong" with her trusting Kohaku, or even that she genuinely didn't believe she could betray her. Having played the game, my impression from her route was that a willing blindness occurred, or from a different point of view, she wanted to believe that she could atone for Kohaku's mistreatment by her father, and couldn't recognize that her efforts to be at least humane to her maid were for nothing. Everyone was fooled, sorta. It was aided in part by the way Hisui acts honestly. It's been a long time and I'm probably wrong, but as I remember it, it seemed Akiha knew at least a little that Kohaku was dangerous, but probably didn't resist out of guilt or a subconscious desire to be punished.
Ah, poor Akiha. Had I the sick desire to keep Traveler going, you would have finally had a fic on this forum in which you're not a baby-eating ogress. She always seems to be the insane, jealous washboard monster, not that she wasn't one in some routes. No you wouldn't have finally gotten a Good End in that either.