1. The text must be read from "bottom to top".
Summorum contains 12 articles of law, in its very end, the heart of the text. They recognize facts and rights, establish rights which are new or were unclear under previous law, and create new obligations.This is extremely important: those 12 articles are the law. Naturally, other points of law apply (general principles, concepts explicitly mentioned in the articles themselves, as well as other applicable canonical aspects), but neither the introduction to the articles (the first part of Summorum), much less the cover letter sent by the Pope to the Bishops, nor any other text may be invoked to suppress or curtail the rights recognized or created by the Supreme Legislator in the 12 articles.
2. Whose interpretation?
Thankfully, the articles of law of Summorum are mostly quite clear. And where they may not be clear, there is a Roman Dicastery ready to provide the appropriate interpretation and probably unwilling to renounce to its recent increment in power, to be specified by the Roman Pontiff in the future, according to his will (cf. Art. 11).The text of reference is and will be only the Latin original. (Currently, there are no "official" Vatican translations, and our own version fixes just a few problems of the Vatican Information Service unofficial translation. One might only hope that the "official" Vatican translation will be adequate.)
3. Summorum is a new "Constitution" of the Roman Rite.
The Supreme Legislator wished to create a liturgical framework for priests and faithful - particularly for priests. It is a "Constitution", not as a theological document, but in the legal sense that it is a foundational law, a law above other laws: that is very clear, for instance, in the extremely important articles 2 and 4 (Masses without the people or "private Masses", with or without attending faithful), and 9, § 3 (free use of the Roman Breviary), which are the very embodiment of the liberation.