Why isn't những/các used before người? Isn't the object a plural noun (i.e. American people)?
Is only American given a separate word for their citizenship or does "British" too? Am I correct to guess that for other nationalities it would be la nguoi and the name of the country?
That goes with every nationality. So "người" + "country's name".
It's a coincidence that My Lai has the word "Mỹ". Mỹ as America in Vietnamese deprived from Chinese 美国 (Mei-guo).