I think there is a mixed use case. I think your Friend Frank can be smart enough to use formal addresses with Prince Peter if they might be overheard, and informal address when there seems no chance of that.

In any actual true friendship, I should think this must be the case, otherwise (speaking as a modern reader) I would feel there is an arms-length relationship where Frank is at all times subordinate to Peter, and that doesn't feel like a "friendship" to me, that feels like Frank is a pet of Prince Peter.

With mixed address, as a reader I can understand the necessity of the pretense in public; but we know privately that Peter considers Frank an equal in spirit, and Frank considers Peter a true friend, he doesn't feel like a servant to a master. I would make the pretense a secret between friends, Frank can even be wildly over the top in his addresses to the Prince in public, perhaps mocking the obsequious emissaries they have both heard in the royal chamber.

"Oh, prince of light and divine inspiration, prince of valor, prince of endless wisdom, prince --"

 

"Just get my damn beer, Frank!"

I think that imparts a little humility for Peter. This gives you chance, with Frank in private, for Peter to claim he knows his title, wealth and privilege are conferred by birth alone, that these were given to him, not earned by him. And though he will exploit that bucket of luck to the full extent he can, he knows his father too well to believe there is one drop of divinity within the King. And by extension, himself.

This makes the prince more relatable to readers, and the formality of address more of an insiders joke (which includes the reader), so to outward appearances it more closely reflects history, but nobody can claim your "enlightened prince" or friendship is impossible. We can't read dead medieval minds, and we can't trust what writing survived; it could all be lies and formalities.

Although these people did not have all the same facts and education as we do, they did have the same level of innate intelligence and reasoning as we do. So your enlightened prince may be an outlier, but isn't an impossibility.

I think there is a mixed use case. I think your Friend Frank can be smart enough to use formal addresses with Prince Peter if they might be overheard, and informal address when there seems no chance of that.

In any actual true friendship, I should think this must be the case, otherwise (speaking as a modern reader) I would feel there is an arms-length relationship where Frank is at all times subordinate to Peter, and that doesn't feel like a "friendship" to me, that feels like Frank is a pet of Prince Peter.

With mixed address, as a reader I can understand the necessity of the pretense in public; but we know privately that Peter considers Frank an equal in spirit, and Frank considers Peter a true friend, he doesn't feel like a servant to a master. I would make the pretense a secret between friends, Frank can even be wildly over the top in his addresses to the Prince in public, perhaps mocking the obsequious emissaries they have both heard in the royal chamber.

"Oh, prince of light and divine inspiration, prince of valor, prince of endless wisdom, prince --"

"Just get my damn beer, Frank!"

I think that imparts a little humility for Peter. This gives you chance, with Frank in private, for Peter to claim he knows his title, wealth and privilege are conferred by birth alone, that these were given to him, not earned by him. And though he will exploit that bucket of luck to the full extent he can, he knows his father too well to believe there is one drop of divinity within the King. And by extension, himself.

This makes the prince more relatable to readers, and the formality of address more of an insiders joke (which includes the reader), so to outward appearances it more closely reflects history, but nobody can claim your "enlightened prince" or friendship is impossible. We can't read dead medieval minds, and we can't trust what writing survived; it could all be lies and formalities.

Although these people did not have all the same facts and education as we do, they did have the same level of innate intelligence and reasoning as we do. So your enlightened prince may be an outlier, but isn't an impossibility.