A well written argument, but you seem to omit the fact that racial differences are...well, different from religious. Push came to shove many times in the US history were blacks fought in every war and still faced discrimination. Whilst in times of peril they were brothers at arms (to an extent) they still had to face racism when they came back home.
The situation with Elizabeth involved a single ethnic group with religious differences.
So even if they fight together its no guarantee of a well adjusted multicultural society, although I should point that attitudes have changed and there is more tolerance of differences nowadays. Even if there is enough racism to counter it.