The commentaries I see that say that engage in all sorts of mental gymnastics to get to both valid and licit.
Since those Bishops actually lack any ability to exercise their office according to canon law they can't be.
Basically what you have is break away groups pretending to be the true Church claiming validity. Mormons in Catholic vestments with their own version of the great apostasy.
I'm not sure what is considered mental gymnastics for you, but I hardly see it that way. I am sure you are referring to the lack of a papal mandate (decreed by Pope Pius XII) when it comes to Sedevacantist bishops being consecrated, so that would make them valid but illicit.
However, how would you suppose that a bishop get papal mandate, when there is no pope? It would be begging the question if you say that it must come from Francis I, because his legitimacy is the very thing in question. Someone who is not a pope cannot give a papal mandate. So what are we to do? Cease Apostolic Succession? Or is there provisions that deal with this very matter?
St. Thomas Aquinas defined law as such, "an ordinance of right reason made for the common good promulgated by one who has authority in that society." Now a law may cease being a law in two ways, either by the legislator abrogating the law (extrinsic cessation) or, due to extreme circumstances, the law is no longer for the common good (intrinsic cessation).
As Archbishop Amleto Giovanni Cicognani, Professor of Canon Law at the Pontifical Institute of Canon and Civil Law in Rome, taught in his commentary: “A law ceases intrinsically when its purpose ceases; the law ceases of itself... the law ceases extrinsically when it is revoked by the Superior...Relative to the first way: The end (either of its purpose or its cause) of the law ceases adequately when all its purposes cease.
The purpose of the law ceases contrariwise when an injurious law becomes either unjust or impossible of observance.”
The underlying principle in Canon Law is that the ultimate law is the salvation of souls. In Pope Pius XII's day, being that there was a legitimate pope that could give papal mandates, his law ensured no one would be led astray by false bishops, particularly from the National Church of China. Now-a-days, however, his law would lead to the cessation of Apostolic Succession, which would be disastrous to the salvation of souls; once again assuming that there is no pope as Sedevacantists claim.
Pope Pius XII stated in his address to the clerical students of Rome on June 24, 1939: “Canon law likewise is directed to the salvation of souls; and the purpose of all its regulations and laws is that men may live and die in the holiness given them by the grace of God.”
So if Francis I is a true pope, then yes Sedevacantist bishops are illicit. If he is not a true pope, however, then they are perfectly licit. So the question remains then, is Francis I a true pope? This is a question that cannot be answered by talking about licit or illicit bishops, because both answers presume the very thing that is in question.
Now that I think of it, I guess being a Mormon in the eyes of the Conciliar Church wouldn't be too bad. I mean it has to at least be better than, say, the Muslims, Jews, Buddhists, Hindus or any number of non-Catholic religions that they love to praise constantly, at least we're Christian right? And it is surely better than those pesky Catholics who commit the sin of idolatry by being obstinate in their doctrine, it must be them that the Lord despises in the Bible, definitely not the pagans. I think we might be able to fit in after all...