Grand Duchess Maria Wladimirovna is a Grand Duchess of Russia by fact of being the daughter of the late Grand Duke Wladimir, former Head of the Imperial House of Romanov and thus de-jure Tsar.
At present, there are no other dynastic Romanovs excepting the Grand Duchess Maria and her only child, Grand Duke George.
I, for one, find it very annoying when certain members of the Romanov Family Association are referred to as Prince/Princess Romanov when such titles are assumed and totally without legal foundation. Oh well 
The marriage of Grand Duke Wladimir Alexandrovich and Princess Marie of Mecklenburg-Schwerin was indeed approved (recognized as equal) by Emperor Alexander II on 16 Aug 1874 (see here). This is a fact. Grand Duchess Maria Pavlovna converted to the Orthodox faith ca Apr 1908 (see here). This, too, is a fact.
The union of Grand Duke Kirill Wladimirovich and Princess Victoria Melita of Saxe-Coburg and Gotha was recognized by Emperor Nicholas II on 15 Jul 1907 (see here). This is a fact. Victoria Melita had converted to Orthodoxy in 1907. This, too, is a fact. The three children born of this marriage were styled HH Prince/Princesses of Russia until the succession of their father to the Imperial Throne whereupon they became HIH Grand Duke/Grand Duchesses of Russia (see here). This is a fact as well.
Upon the death of his father in 1938, Grand Duke Wladimir Kirillovich became the new Head of the Imperial House and de-jure Emperor of all Russias (see
here). His succession was attested to by the next five Romanovs in the line of succession (see
here).
In 1946, Grand Duke Wladimir issued a decree which confirmed the status of the Royal House of Bagration (see
here). This step was taken to assure the father of Infanta Mercedes of Spain that her marriage with Prince George Bagration of Moukhransky would indeed be equal.
Two years later, Grand Duke Wladimir contracted an equal marriage with HRH Princess Leonida Bagration of Moukhransky (already an Orthodox and the sister of the aforementioned Prince George). The decree of the Grand Duke regarding his nuptials can be found
here. As you can see, the bride of Wladimir became HIH Grand Duchess Leonida.
And, in 1953, the couple's only child arrived, Grand Duchess Maria Wladimirovna. Her father's announcement of her birth and confirmation of her Grand Ducal rank can be found
here.
By the way, the Russian succession has been quite thoroughly discussed in the earlier two threads on Maria Vladimirovna.
