I understand that this book may have answered your questions regarding the orthodoxy/unorthodoxy of Dignitatis Humanae, but it is hardly a refutation of sedevacantist apologetics.
Am I to understand from this response that you really have no doctrinal refutation for the sedevacantist arguments (except perhaps on this one issue)?
Sean, not sure if you followed the French debates over sedevacantism (or sedeprivationism), or whether you have read Mgr des Lauriers' "cahiers de Cassiciacuм", but essentially the debate between ED/SP trads and sedes has always come down to the issue of religious liberty.
This is unlike both groups debate with the R&R, which is essentially over ecclesiology.
But back to sede apologetics. Most notable (former) sedes in Europe who came over to the then-indult, now-extraordinary form, did so shortly after being convinced that Vatican II's understanding of religious liberty was reconcilable with Catholic Tradition. The Society of St Vincent Ferrer is one good example. L'Abbe Bernard Lucien is another one.
So yes, it comes down to this one work authored by Dom Basile Valuet, a traditional Benedictine ordained personally by Mgr Lefebvre. The great irony is that he originally set out to write his doctoral thesis from the R&R position of attempting to prove a rupture between Vatican II definition of religious liberty and Apostolic Tradition.