The Russian Empire not longer exist, legally speaking therefore none can be entitle to call himself PRINCE OF RUSSIA, but academically and socially speaking The member of the former imperial family are Prince/ Princess of Russia.
If so then why not Grand Duke / Grand Duchess? Also what is legal depends on jurisdiction. King Constantine of the Hellenes is King for Danes but not for Republican Greeks.
Whether you think that the descendent of His Highness Prince Roman Petrovich are princes or princess or not, is just your merely opinion. To me and to many member of this forum they are Highnesses and prince and princess of Russia.
Opinion is (as online Webster said) "belief stronger than impression and less strong than positive knowledge". I should to say that positive knowledge on this matter is very strong. There is no possibility for these individuals to hold this styles. They are fakes if they use fake titles and you are ignoramus if you believe fakes. Sorry.
Respecting each other opinion is a very important tool in this forum.
Exactly. As like as respecting correct facts.
BTW, Princess Catherine Ioannovna (b.1915) is merely Serene Highness. Poor goddaughter of Tsar-Martyr! She is not morganaut and does not pretend to Highness...