It seems a pretty big omission in a discussion of the US and nationhood to not mention the concept of civic nationalism, as the US is often considered the main example of civic nationalism. If a nation is an imagined community based on a myth of commonality, that myth doesn't have to be based on just biological natality (that's the etymology of "nation", but it's never been the whole meaning - definitions of nations have always appealed to a common heritage and tradition that is broader than just ancestry). The common myth can be one of values or a shared non-biological origin; indeed the belief that most Americans are descended from immigrants is an important part of the national mythology of the US. This is civic nationalism (and the idea of a nation as necessarily about common descent is, by contrast, referred to as "ethnic nationalism").