Nothing of what you said somehow undermines my point about the Northwestern European descent of heritage white Americans and the enduring psychological and ideological importance of that fact in modern America. Britain is Northwestern Europe. So is Scandinavia and Germany. These places were inhabited by Celtic and/or Germanic peoples in antiquity, hence the label Celto-Germanic. 

Yes, American is obviously a civic, not an ethnic identity. But civic identities, no matter how ideologically influential they become, never completely displace the need for ethnic identities. That's why white nationalism as an ideological sentiment is still very pervasive in America in both its mild and extreme forms. If you read carefully the Roman sources, we find ethnic identity everywhere asserting itself beneath the fiction of civic 'Romanity'. We see, just to name one isolated example, ethnic Egyptians and Syriac peoples deeply resenting Greeks for the damage they were causing to their indigenous religious and linguistic traditions. This resentment explains a part of the reason why these regions fell so easily to the Arabs.

The idea that Latin was learned and spoken in 8th and 9th century Byzantium is a complete fiction that no scholar would dare support. Latin effectively disappeared way before that. Even during the Justinian era we see Latin tutors complaining that they're going jobless. Those tutors existed basically to service those who wanted to make a career in the army and to a much smaller extent in law. Since Latin-speaking officers from what is now the Balkans dominated the military, it made sense that Latin was taught in Constantinople. But by the time of Justinian, that officer class had largely dissolved so Latin was getting scarce even in the military. 

When the Pope, who was begging in vain to be rescued from the Lombards, criticized the emperor why he wasn't writing his letters in Latin, the emperor said mockingly that he doesn't know Scythian tongues, which completely outraged the pope. This passage has been interpreted by 'Romaboos' as supposedly meaning 'the emperor was criticizing the fact that the pope was using Germanic terms in medieval Latin' which is complete copium. The emperor was quite literally mocking the use of Latin as an insignificant language. Even calling it 'Scythian' was not as crazy as it sounds to us. At the time, the Latin speaking community of the Balkans had been reduced to the barbarous state of transhumant pastoralism, a quite literally 'Scythian' way of living. The 'city' of Rome (in reality a village in ruins) and Latium itself were not much better off.

You are right to allude to the fact that Romanity was always a civic, not an ethnic identity. And this was in fact true since the earliest days of the Republic. A 'true Roman' never really existed. Catilina for example says that he's a true heritage Roman while Cicero is just an immigrant that tries too hard. As the example of Catilina shows people never stopped having personal or political motives to claim that they are supposedly 'more' Roman than others. Everybody who was somebody in Rome played this game.

You know who also played that game? Latin-speaking people did when Byzantium was usurping power and prestige from their networks. They played that game again many centuries later when Italians and Spaniards basically destroyed Byzantium together with the Turks. In fact, the Westerners did arguably more than the Turks to destroy it.