Could you tell me some greeting expressions that could replace "hello?"

Why does he say "a shop", not "this shop"?

He says "a shop" here instead of "this shop" because he is speaking generally. Even though he is referring to the guy closing up this specific shop, he is speaking generally towards the situation of someone closing up a shop in general.

"So" is appeared twice in this sentence. Could you explain the role of the two "so"?

This actually was a mistake by the speaker. He accidentally said "so" twice. There really isn't a specific role of using two "so." He just stumbled over his words.

Is there a specific word that indicates people who hate others?

Perhaps a casual term for someone who hates others is a "hater" or "haters," especially in reference to famous people. A more formal word is "misanthrope," but this is hardly ever used! Ex: Taylor Swift's haters hate when she has a good time. Ex: Don't be such a hater. Ex: He was a misanthrope.

Is there any difference between saying "in a single night" and "in one night"?

Yes, there is a slight difference. Although they mean the same thing, the speaker has used "single" to emphasize the fact that one night was a very short amount of time. "Single" is commonly used in this way as a point of emphasis. We can understand this as meaning that the task given to Psyche was extremely difficult as she only had one night to complete it. Ex: I ordered a pizza and it's all gone! My family didn't even leave me a single slice. Ex: I finished the book in a single night.

What does "fall apart" mean? Is it a common expression?

"Fall apart" is a common phrase! It has a couple of meanings. It either means something comes apart or disintegrates, or it means losing one's ability to cope emotionally. Here they're using it to refer to emotions! Ex: I fell apart when they played the last song at the wedding. = I cried when they played the last song at the wedding. Ex: Jane's gonna fall apart soon. = Jane's gonna have an emotional breakdown soon.