Oro's true reason for attacking the leaf is the biggest mystery in the Narutoverse.
Kaguya's army, Obito controlling Yagura, Mei reforming the mist, all minor stretches of the imagination, not really mysteries. Same with Hashirama's death, it was a heavy wartime, he could have been fighting for days without rest and got ambushed when he was weakened. These are not 'mysteries' to sweat over.