In regards to whether or not "most" sedevacantists hold that the current ordination rites are invalid, I doubt anyone here can answer that since I don't think anyone on this forum knows most sedevacantists.
I tend to believe that most sedevacantists would agree that all of the rites of the Novus Ordo are of, at least, questionable validity. It has been widely publicized that Father Michael Oswalt received a conditional ordination from Bishop Pivarunas of the CMRI. That indicates that the official position of the CMRI is that the new ordination rites could possibly be valid.
One of the SSPX priests who come to my parish introduced himself during his first sermon by telling us that he was conditionally ordained by the SSPX. He said that he had no doubts that his ordination was valid, but the SSPX wanted to remove all doubt. Whether this is "official" policy of the SSPX or if they had some information specific to his ordination, I don't know.
Even as a sedevacantist, I accept the possibility that Benedict 16 may somehow be a pope that retains sufficient administrative authority to keep the organ of the Holy See running, appoint cardinals, etc., while having no actual jurisdiction over the faith and morals of the Church since he routinely denies the faith and teaches error and has done so throughout his public life. I understand completely that this is a possibility that the Church, when the crisis has been resolved, will have to rule upon. On the other hand, I also believe that when the Catholic Church truly has a new pope, he will order the bishops whom we now call "independent", sedevacantist, SSPX, etc., whose lines are undoubtedly through true pre-Vatican II bishops and whose consecrations back to those bishops were in the traditional rite to conditionally ordain and consecrate all Catholic bishops who remain in communion with him. I say "remain in communion" because many conciliar bishops will abandon a true pope who insists upon returning to the traditional faith.