Unfortunately, you are not allowed to ask multiple questions on enotes, so I have edited your question to focus on the climax of this tremendous short story.
The problem with living life as an illusion is that eventually reality imposes itself and shows the illusion for the untruth that it actually is. This is precisely what occurs in "Miss Brill," as this lonely woman goes to the park on Sunday and distracts herself by watching the various people their and listening in to their conversations. Miss Brill enjoys this weekly sojourn so much because she imagines it to be a kind of massive play, where even she has a part. This allows herself to imagine that she too is important and is significant:
Even she had a part and came every Sunday. No doubt somebody would have noticed if she hadn't been there; she was part of the performance, after all.
She imagines being able to tell the "invalid old man" that she reads the paper to that she is "an actress" and has been one "for a long time." Such rampant escapism allows her to forget the meaninglessness and isolation of her life. Yet, at the climax of the tale, she is forced to accept it. The climax comes when she overhears a boy and a girl talking about her slightingly, destroying her fantasies, and sending her back home in tears:
"No, not now," said the girl. "Not here, I can't."
"But why? Because of that stupid old thing at the end of there?" asked the boy. "Why does she come here at all--who wants her? Why doesn't she keep her silly old mug at home?"
"It's her fu-fur which is so funny," giggled the girl. "It's exactly like a fried whiting."
With these words, Miss Brill is forced to see that this elaborate drama she has created in her mind is an illusion, that her fur that she was so proud of is outdated and she is a lonely woman who is insignificant. This is the moving climax of this story.