> there is still an indefinable "Americanness" common to them all

I believe it is more of self fulfilling prophecy imo. Some quality you treat as American AFTER you learn it is an american accent rather than something you see as american before (or regardless of whether) you even know if it is american

Maybe? But I heard an Orkney accent once, and my mind refused to classify it as Scottish, even though it was coming from a Scottish mouth.