The SSPX have never claimed their own Magisterium. They have always defended the Magisterium of the Church.
This whole issue revolves aroun Bishop Williamson. Bishop Williamson is not the SSPX, he is only one member of that group, one man, one bishop.
The SSPX is made up of four bishops and all the priests. Bishop Fellay is the Superior and he is in charge. He’s the main man.
Bishop Williamson says alot of things that are wrong and controversial, but they are all his opinion.
He is not making his own Magisterium, but is making errors that are wrong that belong to opinion.
The issue is not a new Magisterium, but the erros of one Bishop. Bishop Williamson can be controlled and corrected by other SSPX members and Fellay.
As far as Canonization, I believe it is infallible and so do many Catholics, but it is not that settled. I have read that theologians agree and disagree whether it is an infallible act. I am not so sure that it is a settled doctrine.
From the Catholic encyclopedia:
newadvent.org/cathen/02364b.htm
"Is the pope infallible in issuing a decree of canonization? Most theologians answer in the affirmative. It is the opinion of St. Antoninus, Melchior Cano, Suarez, Bellarmine, Bañez, Vasquez, and, among the canonists, of Gonzales Tellez, Fagnanus, Schmalzgrüber, Barbosa, Reiffenstül, Covarruvias (Variar. resol., I, x, no 13), Albitius (De Inconstantiâ in fide, xi, no 205), Petra (Comm. in Const. Apost., I, in notes to Const. I, Alex., III, no 17 sqq.), Joannes a S. Thomâ (on II-II, Q. I, disp. 9, a. 2), Silvester (Summa, s. v. Canonizatio), Del Bene (De Officio Inquisit. II, dub. 253), and many others. In Quodlib. IX, a. 16, St. Thomas says: “Since the honour we pay the saints is in a certain sense a profession of faith, i.e., a belief in the glory of the Saints [quâ sanctorum gloriam credimus] we must piously believe that in this matter also the judgment of the Church is not liable to error.” These words of St. Thomas, as is evident from the authorities just cited, all favouring a positive infallibility, have been interpreted by his school in favour of papal infallibility in the matter of canonization, and this interpretation is supported by several other passages in the same Quodlibet. This infallibility, however according to the holy doctor, is only a point of pious belief. Theologians generally agree as to the fact of papal infallibility in this matter of canonization, but disagree as to the quality of certitude due to a papal decree in such matter. In the opinion of some it is of faith (Arriaga, De fide, disp. 9, p. 5, no 27); others hold that to refuse assent to such a judgment of the Holy See would be both impious and rash, as Francisco Suárez (De fide, disp. 5 p. 8, no 8); many more (and this is the general view) hold such a pronouncement to be theologically certain, not being of Divine Faith as its purport has not been immediately revealed, nor of ecclesiastical Faith as having thus far not been defined by the Church.’
Beatification is not infallible:
“This general agreement of theologians as to papal infallibility in canonization must not be extended to beatification, not withstanding the contrary teaching of the canonical commentary known as “Glossa” [in cap. un. de reliquiis et venerat. SS. (III, 22) in 6; Innocent., Comm. in quinque Decretalium libros, tit. de reliquiis, etc., no 4; Ostiensis in eumd. tit. no 10; Felini, cap. lii, De testibus, etc., X (II, 20); Caietani, tract. De indulgentiis adversus Lutherum ad Julium Mediceum; Augustini de Ancona, seu Triumphi, De potestate eccl., Q. xiv, a. 4). Canonists and theologians generally deny the infallible character of decrees of beatification, whether formal or equivalent, since it is always a permission, not a command; while it leads to canonization, it is not the last step.”