Belgium never uses the name "Queen Mother"/"Koningin Moeder" for its queens, nor "Queen Grandmother" e.g. as for Queen Elisabeth from 1951.
I believe the official use of "the" implies ther person who is officially performing the role.
After Albert II abdication, Mathilde became the Queen because her husband is the King, no matter how many more kings and queens Belgium had at the time. Albert became HM King Albert (II), Paola became HM Queen Paola and Fabiola continued to be HM Queen Fabiola. After Astrid's death, Élisabeth continued to be HM Queen Élisabeth no matter there were no other Queen simply because she was not the consort of the then reigning King, her son Léopold III.
Of course, when speaking of someone in an unofficial capacity, that's up to the person. If I'm talking to you about Paola I might say "the Queen" several times in my speech since I know you know about whom I am talking about. In that situation "the Queen" would mean "the Queen we are talking about".
A discussion I raised on the British board about the title and definition of Queen Mother leads me to ask about Elisabeth of the Belgians.
Correct me if I'm wrong, but I believe she lost the designation of being THE Queen of the Belgians, once her husband died -- correct? For that distinction would have belonged to her daughter-in-law Astrid, consort of the new king, Leopold III.
Question: did she regain this distinction 18 months later, after the tragic accidental death of Queen Astrid? After all, Elisabeth remained the only queen of Belgium for the next 25 years, just as she had been during the entire 25-year reign of her husband, King Albert I.
Or is it a universal custom to style only an enthroned queen as THE queen of the country, regardless of whether she is the only woman to bear the queenly title? An enthroned queen, by definition, is either a queen regnant (which before 1991 was not possible in Belgium) or the consort of an enthroned king.
As such, Paola would have lost the distinction of being THE Queen of the Belgians, once her husband (King Albert II) abdicated. But at the time (2013), there were two other women in the country also styled with the queenly title. Aside from her daughter-in-law Mathilde, Fabiola (widow of King Baudouin) was still living at the time. So nobody would have been inclined to even think of Paola as Her Majesty THE Queen, anyway: it almost implies that she's the only one.
Well, we have seen how Mary of Great Britain found herself as the mother of a bachelor king, for 11 months in the fateful year of 1936. So at the moment, she remained the only woman in the country styled with the queenly title. In this, she was like her 19th century German namesake, Marie of Bavaria.
But like I said, I'm not entirely certain if THE Queen implies an enthroned queen or simply the only woman in the country styled with the queenly title.