(I have edited your multiple questions since only one is allowed per eNotes post.)
Although Scout is pretty much clueless about why Atticus is reading in front of the jail and why the other men are there, she can sense that something is wrong. She sticks by Jem when he stands his ground to defend Atticus, and she does the same for Jem when he is manhandled by one of the Old Sarum boys.
"Don't you touch him!" I kicked the man swiftly. Barefooted, I was surprised to see him fall back in real pain. I intended to kick his shin but aimed too high. (Chapter 15)
But aside from her normal feistiness and loyalty to her father and brother, Scout shows a tender, kindhearted nature not always exhibited in the novel. When she recognizes Mr. Cunningham, the guilt about her previous treatment of Walter Jr. kicks in; she also remembers Atticus's advice
... that it was the polite thing to talk to people about what they were interested in..., (Chapter 15)
so she begins an innocent conversation with Walter's father. Her naive talk about his entailments and Walter being "a real nice boy" soon shames the men into abandoning their plans to lynch Tom Robinson.
We see that Mr. Cunningham is quite different from the equally poor Bob Ewell, who seems to take pleasure in Tom's situation and who will later try to add murder to his list of sins. Atticus has previously told Scout that Cunningham is poor but honest, and he displays a sense of moral character when he "squatted down and took me (Scout) by both shoulders." Calling her a "little lady," Cunningham calls off his ill-laid plans (which were probably initiated by the help of alcohol) and returns to his senses.