The damage was in the aftermath, when the progressives in the Church went about promoting abuses in the “spirit of Vatican II”.
But as you point out, there were decades of intellectual rot that led to that point. (Pointing to the Council as the cause of our current state is really a very simplistic way to avoid acknowledgement of the other problems that preceded it.)
What do you mean by “valid council”? Validly convened? There are some who dispute the prudence of calling a council, yet refusing to address the primary issues of the day, which would have been to condemn atheistic communism and modernism. On the contrary, the subject of communism was taken off the table from the get go. Pope John XXIII was either a tool/fool or he was in on the revolution which was driven by the free masons in the curia.
Or do you mean that the progressives wrote a bunch of documents and then forced the Holy Spirit to bless their work? That is, a cabal did evil to the Church and then said the Holy Spirit inspired it?
Lefevbre had significant problems with several of the documents, especially the one on religious indifferentism, Dignitatis Humanae.
It should be noted that VC II actually had a named spirit. That is, the “Spirit of Vatican II” is separate and distinct from the Holy Spirit. It is a fallen angel.