Do you happen to know when Charles X elevated all dynastic members of the House of Bourbon to Royal Highness?
It was the reigning King Charles X of France and Navarre who elevated all dynastic members of the House of Bourbon to HRH where they previously had been HH. He did so because many of the newly elevated dynasties now had equal or higher titles than the members of his own august dynasty.
When Grand Duchess Charlotte married her first cousin Felix he was incorporated into the Luxembourg nobility as HRH prince Felix de Bourbon de Parme. So his male line descendants are entitled to the HRH on two grounds. As members of the House of Bourbon and as male line descendants of someone incorporated into the Luxembourg nobility with that style.
The style and naming was copied when the grandchildren of Felix' brother Xavier were incorporated in the Dutch nobility. Because of the Dutch noble laws Hugo Klynstra, the son sired by HRH prince Carlos de Bourbon de Parme, titular duke of Parma and Piacenza, before his marriage, was able to force his biological father to accept him as a legal child resulting in a change of name and style. The young man now is part of the Dutch nobility as HRH prince Hugo de Bourbon de Parme. He is however not part of the legitimate Parma-branch of the Bourbon family or of the dynasty at large.
As I understand it, only the sovereign and heir were Royal Highnesses, with cadet members being Grand Ducal Highnesses. But the two eldest daughters of Grand Duke Guillaume IV succeeded to the throne as reigning grand duchesses, with two other daughters marrying into the royal rank. However, both husbands came from non-reigning dynasties (Saxony and Bavaria). Nevertheless, was the Royal Highness style legal everywhere -- in particular, recognized in Luxembourg?
It has occurred to me that a similar thing happened later on, when Charlotte married her first cousin, Prince Felix of Bourbon-Parma -- who, despite being a member of a long-deposed dynasty, was a Royal Highness as a male-line descendant of King Louis XIV of France (itself a long-abolished monarchy). Accordingly, their own male-line descendants have been elevated to that style.
Nevertheless, isn't the style of dubious validity, in light of the fact that it comes from the non-reigning royal house of Bourbon? Yet, it is being applied to members of a reigning grand duchy ...