“... that it was insanity for Japan to have gone to war against _this many_ nations.”
Like the story of “The Emperor has no clothes”, I’m guessing not too many people said no to the Emperor.
And for all of the planning, iirc they had to quickly improvise a steel kitchen table for the signing.
Yamamoto and the Navy were strongly against going to war against the US, but the admirals knew there was a good chance they would be assassinated if they didn't go along. Japanese politics weren't very civilized in that era.