One of the things I would say about generic American culture is that it’s still a little unsettled. The upshot is that I don’t know the “right” answer to a single one of your questions (except the last one, sort of) – and I get the impression no one else does either.
My point is there isn’t a generic, American, correct answer to any of them. (and to many more questions, I’m sure) I’ve read at least one thread about each of those subjects, and the replies go from one extreme to the other. One person believes it’s unspeakably rude to ask guests to remove their shoes, and another thinks the guests are being rude because they didn’t do so automatically. Someone believes cash wedding gifts are tacky, and someone else has never heard of giving anything but cash.
The trouble is, there are plenty of Americans (me among them) who are not steeped enough in one regional culture to even know the local right answer. Imagine someone who has moved around a certain amount as a kid, from one middle-class suburb to another. That describes a lot of us, and what we tend to have as a culture is the generic American middle-class one. It’s no surprise that we don’t know the regional norms for all these points of etiquette. And moreover, these norms are not themselves that stable, since they’re affected by in- and outmigration, and broader generational and cultural changes.
Often I wish we did have more consensus on these etiquette-type matters. It would make life easier for everyone, and save us a certain amount of embarrassing social fumbling.