Since the flames of the gas chambers and the flames of the battle extinguished his true belief in god, he is saying that he will never forget. He will never forget the pain and suffering and his lost loved ones and all the pain he survived through.
It grew louder—louder—louder! And still the men chatted pleasantly, and smiled.
Correct. If the police officers could hear the sound, they likely would not be chatting pleasantly and smiling, which suggests that the narrator is hearing the wild beating of his own heart.
Lending a friend a huge amount of money is not really recommended, because they may ask you over and over and over again, causing you to forget how much she owes. She may also spread the word, making you vulnerable to more thefts.
The Cunningham boys were troublemaking teenagers who were arrested with Arthur "Boo" Radley. Boo was taken home and locked away from the world by his father but the Cunninghams were sentenced to attend a state juvenile hall of the 1930s. The irony here is that the school actually gave them great education and one of the Cunninghams went to college to become an engineer.
Hope this helps =]