You're correct so far as that it wouldn't be dynamic when compared to the pagan versions (York, Novgorod, Uppland, etc). However, different CoAs based on culture based on culture does exist in the game in two instances: Lombardy --> Italy and Avaria --> Pannonia.
The changes are not dynamic though; they take place immediately when a non Lombard takes Lombardy or when a non Avar takes Avaria. They are irreversible and the effect is signified with events ("The Kingdom of Italy" and "Fall of the Avars" respectively). The same could easily be accomplished with England changing whenever a non Saxon acquires it.
(Mechanically, I think it's managed by having two completely separate titles having their title history copied and de jure territory automatically transferred. If it's separate titles, then they might have different associated cultures: meaning that future Liberation Revolts are English rather than Anglo-Saxon?)
Possibly, but since this feature has been in the game since Charlemagne and hasn't been expanded since so I find it unlikely. Additionally, of your examples only the non Capet french flag is in any way reasonable but is in a slightly different ballpark since it's dynasty specific rather than culture specific. Having realms receiving CoAs based on the ruling dynasty is a Muslim (and Nomadic) feature.