If O-Kagachi deals lethal damage to a blocking creature and also deals damage to its controller, that blocking creature is destroyed before a target is selected for O-Kagachi's last ability. It can't be exiled.

A player attacked you if that player declared a creature as an attacking creature and that creature is attacking you. If that player attacked only planeswalkers you control, that player didn't attack you. If that player's creatures were attacking you only because they were put onto the battlefield attacking or only because you were reselected as the player they were attacking, that player didn't attack you.

O-Kagachi's last ability exiles only one nonland permanent, no matter how many creatures attacked you or how many times that player attacked you during their last turn.