Right so you do have a few points but... the Imperium absolurely does enforce a national identity and maintain a constant state of conflict against internal and external enemies.

National identity-wise, the Imperium believes in the idea of a united humanity centered around the Imperium and its ideology (first the Imperial Truth and then the Imperial Cult). Those who are not seen as part of that identity - Xenos, mutants, those following the wrong religion - are ruthlessly exterminated, as are those humans who reject the idea of the Imperium. If that is not radical nationalism, I don't know what is.

As for the constant state of conflict against internal and external enemies... I mean, that's what the entire setting is about.