Sedevacantism is an old error, and the SSPX has refuted it many times, with very solid doctrinal and theological reasons, which it is unnecessary to go into here. Let's come to religious liberty, because the heresy of indifferentism it has led to in practice (as the Popes of old with remarkable prescience forewarned) still afflicts countless millions of souls in the Church.
I know there've been attempts to show the text of Dignitatis Humanae is only ambiguous, and ultimately reconciliable with Tradition, though even some of these condede its practical application has been flawed. But there have been other very exhaustive studies showing otherwise.
Pope Gregory XVI in
Mirari Vos, Pope Pius IX in
Quanta Cura, Pope Leo XIII in
Libertas and also in
Immortale Dei] and finally Pope Pius XI in
Quas Primas and Pius XII in
Ci Riesce express the traditional teaching. This last sums up two important principles, relating to why tolerance can sometimes be allowed, but never liberty as was traditionally defined.
First: that which does not correspond to truth or to the norm of morality objectively has no right to exist, to be spread or to be activated. Secondly: failure to impede this with civil laws and coercive measures can nevertheless be justified in the interests of a higher and more general good.
A false religion is not a good to which liberty is owed as a matter of justice, but on the contrary is an evil even the toleration of which requires a proportionate cause, and can only be permitted in the interests of a greater general good, as Pius XII goes on to say.
The major, though not only, heresy widely held in Conciliar circles relating to this matter is that religious liberty includes a natural right to error, a right to choose a false religion. This heterodox understanding was pan-universal in the newChurch.
+ABL had asked long ago, "Could it, please, be explained to us how man can have a natural right to error?"But now, after almost 50 years of giving an impression otherwise, the Roman authorities themselves concede in the doctrinal talks with the Society that yes, in reality, there is no right to error. +BF said, "In our talks with Rome, they clearly said that to mean that there would be a right to error, or right to choose each one his religion is false. "
Whether DH actually says there is or is not a natural right to error is another question, but the undeniable fact is that almost everybody thought it did - and they were wrong, as even Rome now(though not yet as openly and as firmly as the Society would like) admits. This is another proof of the possibility and necessity of resistance to many of the private and non-infallible acts of the Pope.
And the failure to resist would have had everyone thinking there really is a natural right to error. I can show you, dear Pete, several apologists who before this had publicly stated that error had rights, contrary to the traditional teaching of the Church, and rebuked traditional Catholics for saying that that was impossible and heterodox.