D.Pedro took arms against his father - but I guess you would not consider that treason - and provoked the secession of Brazil.
He became a foreign prince, and, therefore, unable to succeed to the portuguese throne (did you know that ?)
Even if D.Miguel was an usurper (and I stress the Even IF) he was the de facto King of Portugal, recognized by the people and the foreign powers.
If you hadn´t realized that before, even usurpers can become kings and sovereigns.
History is full of examples:
Napoleon usurped Louis XVIII's throne after the 1st restoration.
Do you deny him the title of Emperor in that period ?
Felipe II, as he used to say, through marriage, bribery or conquest, usurped the portuguese throne in 1580, after a mock-up Cortes, when he was not by far the legitimate candidate.
But he became King of Portugal.
In 1640, the duke of Bragança, "usurped" the throne from the Habsburgs, becoming KING D.João IV .
The Crown-Prince D.Pedro usurped the Brazilian throne from his father and became Emperor of Brazil, right ?
In Naples, the Bonapartes and Murat usurped the local throne and were recognized as kings.
Henry Tudor usurped the english throne (to which he had no legitimate claim), after defeating Richard III who had usurped the throne from his nephews.
Vittorio-Emanuelle II usurped the thrones of Tuscany, Modena, Naples, Parma and even the Papal territories.
Vittorio-Emanuelle III usurped the albanian throne and proclaimed himself King of Albania.
He also usurped the Ethiopian throne and proclaimed himself Emperor.
Napoleon Bonaparte usurped the spanish throne and sat there his brother Joseph.
They were all recognized as sovereigns of their usurped countries so your argument is not valid.
Usurpers - and I am not saying D.Miguel was one - can become Kings and be styled as such.
What is your basis for styling him thus? He was a usurper, not the rightful occupant, of the Portuguese throne ...
548