Religious freedom can not be a dogma because it is based on natural law, not Divine Revelation.
As far as I'm aware, the Church's infallibility only extends to the contents of divine revelation and truths closely connected therewith.
But it has been taught repeatedly since John XXIII, with Vatican II even saying "this holy synod declares that man has the right to religious freedom...", so it is clearly infallible.
Not necessarily. The buzzword for infallibility (at least since the 19th century) is define
. Of course, no specific buzzword is necessary for the a pope or ecumenical council to exercise its infallibility, but had Vatican II wanted to make it clear that it was speaking infallibly, it would have used that word. Also, I don't think that just because something has been taught by a few (in this case five) popes is is necessarily part of the Church's ordinary and universal magisterium.
And Paul VI said that Vatican II was "doctrinal and pastoral" but not "dogmatic"; he also said it was binding on the Church, since the Church was teaching with an "authority that could not be called into question". THerefore the SSPX has no right to be preaching dissent just because they can't understand the document
But Paul VI also explicitly denied that Vatican II had exercised extraordinary infallibility.