Why did Mumei "need to go to the toilet"?
Because it would be weird if she separated from Biba at any other point. They were closely followed and in open while going in. So it would raise an alarm if they tried something funny. 


  And even if her absence had drawn attention... all of the witnesses would've been either murdered or kidnapped by Biba.


The point was not to draw attention until gates are open. If someone like Biba was just walking around it would definitely raise alarm. Someone like Mumei can easily stealth around and stun anyone who sees her. There is also added benefit of Biba staying with the "governor" distracting him.

And because only women and children were allowed to follow him inside (I assume because they believed women and children won't be a threat to soldiers), Mumei and the other woman were clear choices.