Why did Mr. Cunningham's mob leave? What was the importance of Mayella's bruises being primarily on the right side of her face?

The children, Jem and Scout, have gone up to the jail and see Atticus sitting in the jail house door with his feet propped up.  The mob that gathered at the jail house was an angry mob that smelled of staleness and whiskey. They had arrived in cars.  The men are speaking in soft tones probably out of respect for Atticus.

The men try to get Atticus to get out of the doorway.  Atticus tells Walter Cunningham to go home.  Scout runs to her daddy pushing through the crowd.  Atticus tries to send Jem and the children home.  A burley man had grabbed Jem and Scout in reaction had kicked him.

The crowd continued to demand Tom Robinson. Scout sees Mr. Cunningham and starts talking to him about some of the things she had heard Atticus discuss. When Mr. Cunningham did not respond, Scout had asked him if he had not recognized her. She starts talking about Walter.  Mr. Cunningham then had signaled the crowd to dissipate.  The humility of the child trying to encourage small talk with her classmate's father had brought Mr. Cunningham back to reason.

The side of Mayella's bruises is significant because one of Tom's hands is useless.