@Nimroth It is not as if you can just randomly come up with uses of words just because the specific word doesnt entirely fit into the translation. The word "emperor" is meant for males. You cant just say "man waitress" and you cant say "man hostess" eighter. That word is gender specific! The word "Emperor" is meant for males. 😅
If you want to use it like that, then it means that it is not english! Instead it would be your own, made up, version of english. (like the Nadsat in A Clockwork Orange)
The expression "empress regnant" does suggest that the empress is the actual ruler of the country. In the other cases the empress is just the wife of the Emperor without much political power.
The only case that is might be used like that would be to make it seem ironic, which would be an artistic addition from the translators to the story. It does not seem that way here. It is most likely a mistake, which happens quite often even to the very best of translators! 😀
Gl