A general rule, if no style guide will tell you: don’t hyphenate unless you have to. (By the same principle, don’t capitalize unless you have to.) If the house style sheet calls for a hyphen, you have to. If it’s in the house dictionary, you have to. I’d make an exception for long quotes: if a long quote breaks house style, then you let it. You can query the author about those or conduct your own fact check. I can still be amazed by what is available online.
CMS provides a long, thorough, and sensibly worked out system for hyphenation. I wish everyone would follow it for hyphens, but nobody does, nor does any dictionary.
When a hyphen question isn’t resolved by house style or the house dictionary, what do you do? First of all, some compound modifiers are animals well known and never need to be hyphenated: health care system, day care centre, and similar. So, don’t hyphenate those, and add those to your own style sheet. Now what about triple modifiers, like health care related fields, day care centred policies, and so on. It isn’t quite so simple. I’ve seen some authors (and editors) apply double hyphens: health-care-centred policies; day-care-centred learning. I prefer the alternative: health care–centred policies, day care–centred learning. In other words, follow the compound with an M-dash. Then breath a sigh of relief when you encounter a book or magazine that does the same thing, like The Atlantic.
That still leaves a broad swath of greys. Is your steak well done or well-done? Hyphen or not? I’d say it depends on whether the double compound follows the noun or precedes it. Hyphen if before: “a well-done steak”. No hyphen if after: “I prefer my steak well done.” Some would contend that “well” in both situations is an adverb and so should not be hyphenated in either instance. The bunfight isn’t worth joining. Add your choice to the style sheet and move on.
United States or US?
This is a tricky one. You’ll know soon enough into the book how often the question is going to come up, and best you make a decision early. What does the house style sheet say? Probably nothing, because the question is too wobbly. Next question: what does the author do? If he consistently uses US, you’d better think long and hard before you interfere. Next question: what is the context in which the term is used? American is a “soft” form of US, you can put it that way. So it’s the US Department of Agriculture but American farmers, the US Congress but American politicians, the US Army but American soldiers. US is official and political, American is social and cultural. If the author veers too far from that principle (I almost typed “basic principle,” but hold on, I thought, all principles are basic), then take a breath and intervene. All of that goes far to shrink the grey zone, though without eliminating it. You triangulate between house style (which is best treated as a fairly firm baseline), the author’s preferences (which you have a right to evaluate), and your own judgment (which you will have developed, if you’re any good at your craft).