First of all, thanks for the mod and all your hard work on it.
I would like to share some thoughts on naming conventions in Rus:
1) Since you want to localize titles of the councilors, here is a proposal for the corresponding ones in Rus:
"Chancellor" = "Dyak", "Marshal"="Voevoda", "Steward"="Tiun", "Spymaster"="Tayny sovetnik", "Capellan"="Duhovnik".
2) "Veliky boyar" for the count-level title sounds completely strange, "Udelny knyaz" would be much better (i think that spelling "knyaz" is better than "kniaz"). That is "Count"-"Duke"-"King"-"Emperor" would be "Udelny knyaz"-"Knyaz"-"Veliky knyaz"-"Tsar". The spouse of a "knyaz" is "knyaginya", not "kniagini" (I think we should have the female consorts being called just "knyaginya" for all three levels of "knyaz" titles). A female consort of a "Tsar" is "Tsaritsa". If a woman is reigning, then the titles would look as "Udelnaya knyaginya"-"Knyaginya"-"Velikaya knyaginya"-"Tsaritsa". Then baron would be "Boyarin", bishop would be "Arkhierey".
2) Since you decided to have de jure "kingdoms" in Rus based on later division into roughly Novgorod/Halych-Volyn/everything else, I would suggest naming this "everything else" "Vladimir-Suzdal". This is meaningful since at the time when Novgorod and Halych-Volyn became sort of separate from the rest of Kievan Rus, the center of power in this rest of Kievan Rus was in Vladimir/Suzdal, and the "knyaz" in Vladimir was called the "Veliky knyaz". Also Novgorod at that time was already an oligarchic republic, like Venice.
This would, of course, mean changing the borders of the kingdoms in a corresponding way (i.e. transferring the counties around Vladimir to this Vladimir-Suzdal kingdom).
3) I would suggest using "Russkoe tsarstvo" (= "Tsardom of Rus") for the Empire-Level title.