Maybe "Jump" was on the radio, when Ocean wrote that part of the script...I also thought that "van" was a typo. Ocean might have have been dreaming of the comet.
Yes, the time of Karl der Große was quite a while back, "just" over 1200 years. While the bad relationship between "East and West Frankia" for most of the time since then has surely played a role that some nobility ranks never spread, I think the later massive centralisation of France also played a role. Even back at the time of the Stem Duchies which would become the HRE/German Empire, there were ranking structures for the nobility.Well, there's a hierarchy between them. For instance some of them, but not all, are "pairs de France", which gave them the right to sit in the Parliament, like Roman patricians had the right to sit in the Senate. If I remember well the rank of archduke was created by an Austrian emperor, and it didn't spread to France, since relationships between the kingdom of France and the Empire never were very good (after the reign of Carolus Magnus). It must be a similar story for the rank of grand duke.
It could even be an "addon" to the woes of Amber. Remember Bella said she was the worst thing which could happen to her Mom. While every noble (and commoner) family has their black sheep, many of them do not want to be so.It might be a reason why it's a huge problem for her to fall in love with a commoner . But sometimes, if you kiss a toad, a prince suddenly appears.
As for falling for a commoner can go both ways. The really old noble families are comparatively relaxed on many things, more than younger ones or those who have to represent, esp. the mainlines.
If, just as a theory, Bella is in line for the title, this might be one additional reason why the only "throuple" possible with her is Nami. For instance, taking Ayua (bestie) and Nami (sis-in-law) with them back to the old country would be very unproblematic.