I know what you're going to say. Marcel Lefebvre and all the other (except four) Council Fathers made a big mistake by signing a blank cheque. We can, with hindsight, see that. To which I will reply that 'hindsight' is a standard of judgement by which few of us would be happy to be judged. But more: the cheque wasn't blank. Sacrosanctum Concilium deliberately made the cheque unblank by ordering "There must be no innovations unless the good of the Church genuinely and certainly requires them; and care must be taken that any new forms adopted should in some way grow organically from forms already existing." Blame the plotters who acted as if the Council gave them a Sartrean carte blanche, not the Conciliar Fathers.
But does this mean that Eucharists confected with Eucharistic Prayer II ... and Orders conferred according to the new, corrupted Pontifical ... are invalid?
It most certainly does not. There are four reasons for this
(1) Do-it-yourself 'theologians' fail to understand that for mere naked validity, the Church requires very little. I have written about this often before (Search Engine?). That highly iffy Eucharistic Prayer, and that deplorable Prayer for consecrating bishops, do contain all that is necessary for validity. They are deplorable and iffy in our Latin Church because they abandon the theology as well as the ancient and noble traditions of the Roman Church; traditions which are as sacrosanct for us as Byzantine and Oriental formulae are to the adherents of those rites. And because the Roman tradition is, in any case, more ancient and venerable and beautiful. But 'Deplorable' does not in any way feed into 'Invalidity'.
(2) Those formulae (a) had for centuries been used by dissident communities in the East whose Sacraments the Roman Curia had by its very ancient praxis always regarded as valid; and (b) when groups left those dissident communities and entered into Full Communion with the See of S Peter, Rome ordered their ancient rites to continue to be used. Nobody was reordained!
(3) Those rites continue to be used to this day both by dissident and by united (Catholic) Orientals. Can it seriously be argued that when a 'Monophysite' Coptic bishop is consecrated, he is truly made a bishop, but that when the same prayer is used on a Latin Rite consecrand, it doesn't "work"? The 'rigorist' stance so absurdly, so gleefully, adopted by sedevacantists means that when the dissident Patriarch of Alexandria consecrates a bishop, Hooray!! The rite 'works'! But when a Latin bishop is consecrated with the same words, God goes all severe and says No.
(4) If there were radical verbal inadequacies or ambiguities in the forms presented by the post-Conciliar Pontifical, we would need to remember that usage and situation can pinpoint and narrow the referentiality of words. So if it is laid down by a particular linguistic community that the words "governing Spirit" shall henceforth, when used by them, refer to the Holy Spirit by which the order of the Episcopate is conferred, then, when the words are (at least, thereafter) used by them in that context, that is what those words do mean. British Acts of Parliament commonly lay down what particular terms within the Act shall therein mean. And they do so mean.
There are ecclesiological problems for all of us when a pontificate goes off the rails as badly as this one has. But the sedevacantist claim that 'Conciliar Orders' are invalid is a facile and cheap dodge, a short-cut, to avoid the hard work involved in working through the real problems. And to do so by scaring good Christian people. It has more than a little something of the Enemy about it.