Thank you for all the details. We have a similar hierarchy in France, except that there are no grand dukes nor archdukes, only dukes.Hi! That depends, "von" is the most common "Adelsprädikat" among some others like "zu". It can show royalty, but it is a general nobility marker.
From Ritter (Knight) upwards. Next are the Barons, which are rarely called this in Germany, Barons are typically called Freiherr/Freifrau/Freiin instead of Baron/Baroness. Then we have the Graf/Gräfin (Count/Countess) with several subdivisions such as Pfalzgraf. Next up are the Markgraf/Markgräfin (Margrave/Marquess), then the Herzog/Herzogin (Duke/Duchess). The Herzöge too have subdivisions (practically all other ranks too, this is just an overview), like Großherzog (Grand Duke) or Erzherzog (Archduke). A special branch is the Fürst/Fürstin which is called Prince/Princess in English, which is a fitting translation, but the nobility ranking differs between German and English ranking here. Then we have the Prinz/Prinzessin (Prince/Princess), the König/Königin (King/Queen) and finally the Kaiser/Kaiserin (Emperor/Empress).
Without Bella´s family rank making it clear, much is possible. I do not think her family to be among the uppermost ranks, because nearly all high nobility families are quite wellknown by name and she would be living in Germany or at least would return to Germany one day. Because if you are "old Blue", that is not in question. Even among the lower ranks this is the usual way. It might even become a topic in the story, if Bella is from old nobility.
I've checked the script, there are a few places where Her Grace Bella is called "van Halen".