The corpse of the snake is not a hint at an unsuccessful ritual. Like I said, he could just take that form before he does the ritual. He could have completely failed at even beginning the ritual or not even attempt it. You're assuming a false dichotomy of the ritual happened and failed or the ritual happened successfully. There's another possibility that the ritual didn't happen at all and Oro just took that form to fight.
I'm saying Kabuto didn't know if the ritual happened at all and there's no proof there for him to "know" that it happened. That is until Sasuke told him.