I disagree with Filippo, the first sentence can make sense, but there is probably a complementary sentence before it, and it's written in a weird way. The key point is that the word "American" there is an adjective, not a noun. What kind of 'need'? An 'American need'. Here's a hypothetical prelude to that sentence: "(There are many American needs: the American need to buy, the American need to be famous, the American need to hustle. And) Just as American as the need to buy, social observers say, is (the American) buyer's remorse." For the other two questions, you'd need to post the full sentences, because it's hard to see the meaning without context. I can only hope that made things less confusing

In practice, none.

I think that "it" in those cases is pointing to a "this" or "that" before you know what kind of thing or person it is. What's this? It's a pen. What's this? It's a dog. What's this? It's a human. What's this? It's me. "This thing" you are touching is me. Oh, who are you? I'm Gui. You're Gui? That is right. "That thing you said" is right. It's right.

Also, it is very typical to see the expressions "tear it up" or "rip it up" which mean the same as how Alex defined "to shred." Shredding tends to be used more when the task is done through the help of a machine. Besides paper you also shred cheese, and you call it "shredding" because you use some tool or machine rather than your bare hands.

Ok, I won't tell you.

Well, according to your concept of "imagined reaction," the first example ("could have used them for a bed") doesn't fit into that. The "could" shows a possibility, but more from the perspective of the narrator than the character. The character did not think of using the wood stumps as a bed, on the other hand he did imagine leaning against the pillars.