The complete letter is in Navarrete. In the 1906 Clark edition of Pigafetta, a lengthy note 119 (pp. 230-234) contains a translated excerpt (p. 233) in which Brito describes the mutiny. The editor points out that “Brito's account of the mutiny is very brief and unsatisfactory.”
According to Navarrete, an Antonio de Brito was captain at Terrenate, and ordered the seizure of the Trinity. Presumably he interviewed the crew, got the facts mixed up, and subsequently sent his report to the King of Portugal. Among its errors are the following:
Although Brito misplaces the scene of the San Julián mutiny at Rio Santa Cruz, and gets the time of de Cartegena's arrest wrong, perhaps the final point above is correct. If so, it is about the only explantion of de Cartegena's banishment that makes sense. For the punishment to have effect, it would have to take place as the fleet departed, not while it remained for the winter. And, the San Julián area was inhabited, while Santa Cruz was not.
Of later historians, in Justin Winsor's Narrative and Critical History …, Edward Everett Hale (author of The Man Without a Country) seems to support this possibility. He writes (vol. II, chap. IX, p. 599):
“Juan de Carthagena and the priest Pedro Sanchez de la Reina were … sentenced to remain when the ships sailed. This sentence was afterwards executed.”