1) The following sentences support the idea that Helen Grey is an unpleasant person: "(...) and say your shrewish say", "don't strain the cord until it snaps", "don't split the sound heart with your wedge" (she secures her heart); "Helen Grey, is that a reason to be proud?", "but you so miss that modest charm" and "stoop from your cold height", "come down you must perforce some day".
2) The sentence that shows correct use of an em dash is the following one: Susie has been invited to go on a trip to the Yukon Territory – her favorite place – and Eastern Alaska. In this case, the em dash has been used in place of commas.
3) The sentence that shows correct use of an em dash is the following one: We're hosting a party – you are invited, of course – later this month. In this case, the em dash replaces the parenthesis.
4) The sentence that shows correct use of an em dash is the following one: By the end of the story – no one could believe it – the main character was dead. In this case, the em dash has also been used instead of the parenthesis. The em dash serves to emphasize the fact that no one could believe that the main character was dead.
5) The text from "Helen Grey" that explicitly explains a more important quality than beauty is the one that reads "but so you miss that modest charm". She is both beautiful and charming.
6) The line from "Helen Grey" that suggests that Helen has an unrealistic sense of herself is the following one: Stoop from your cold height, Helen Grey. She believes herself to be the best, she is too good to others.
7) The text from "Helen Grey" that explicitly describes Helen's behaviour is the following one: Take heed, you yet may trip and fall. This means that Helen should reconsider her attitude towards others because what goes around, comes around.