Maria Vladimirovna's rank is only disputed by the two elderly sons of Roman Petrovich and several descendants of Grand Duke Alexander Mikhailovich.
Regardless of whether or not you accept that HIH Grand Duchess Maria Vladimirovna of Russia is the Head of the Imperial House of Romanov, the fact is that the vast majority of the Royal Houses of Europe (specifically: the Spanish Royal Family, the Belgian Royal Family, the French Royal Family, the Greek Royal Family, the Portuguese Royal Family, the Bulgarian Royal Family, the Serbian Royal Family, the Romanian Royal Family, the Prussian Royal Family, the Wurttemberg Royal Family, and the Albanian Royal Family) as well as the Russian Orthodox Church and the Russian Government recognize her as such. That is what matters.
Sometimes it seems as though most people who oppose Grand Duchess Maria Vladimirovna's role as Head of the Imperial House only do so out of a deep-seated and frankly irrational dislike either of her or of her ancestors, and not because they can get the Fundamental Laws to support their claims.