Instead of arguing about whether
Великий князь should be rendered Grand Duke or Grand Prince (as seen above the terms do overlap to some extent), we should rather discuss whether
Император и Самодержец Всероссийский should be rendered Empeor and Autocrat of all the Russias, Emperor and Autocrat of all (the) Russians or All-Russian Emperor and Autocrat? (The latter being the literal meaning in Russian.)
I know that the Imperial Government used
empereur et autocrate de toutes les Russies and "Empeor and Autocrat of all the Russias" as the official translation in diplomatic handbooks etc., so it's merely an academic argument. But the fact is that all three translations have been used at some point, in Latin, German, French or English. See
this learned discussion on alt.talk.royalty about Peter III's Latin title for some interesting background.
If boys, girls and apple pie can be All-American, a former German reactionary association Alldeutscher Verband and the Swedish premier football league Allsvenskan, why can't Nicholas II be All-Russian Emperor and Autocrat?