I agree with most of your opinion, except in one aspect: Because Portugal has been an independent state for a long time (except the 1580-1640 period), and in this TL, the Portuguese Civil War was ended with a victory for the monarchists, the Portuguese should choose a new monarch from the two contenders for the throne, Queen Antonia and Princess Isabel of Braganza-Borbon dos Sicilias (daughter of Emperor Pedro II of Brazil). (I personally think Queen Antonia could actually refuse the offer, just for the sake of the nation of her birth.)
The Brazilian Constitution of 1824 specifically stated the impossibility of gather under the same person the thrones of Brazil and Portugal, to prevent that in that hypothetical dynastic union, Brazil returned to be considered as a colony of Portugal.
Furthermore, the dynastic lines were separated when Pedro I of Brazil and IV of Portugal decides that his eldest daughter, Maria II, was his successor in the Portugal's throne, while his only son to be the new emperor of Brazil by the name of Pedro II.
Therefore, for the succession rights about Portuguese Crown fall on the descendants of Pedro II of Brazil, it should exhaust all lines of succession from Mary II; and Infanta Antonia of Braganza was her second oldest daughter who survived infancy (his eldest daughter who survived infancy was the Infanta Maria Anna of Braganza, who renounced his dynastic rights when he married a prince of Saxony and died in 1884; and in the hypothetical case that she had not explicitly renounced her rights to the Portugal's throne, it would not surprise me that their relatives and German royal family forced her heirs -who were obtained subsequently the dynastic rights to the throne of Saxony- to abandon their claims with so that their aunt Antonia was crowned as Antonia I of Portugal and she could fulfill the dream of Iberian unification, and thus obtain a stronger ally for the German Empire).