The simple answer is that they have "supplied jurisdiction" as opposed to ordinary jurisdiction. This essentially means that "in an emergency we can break the rules" i.e. epikeia. So we are waiting for our confessions and marriages to become valid in the future, on the presumption that the future Pope will retroactively give jurisdiction to the sedevacantist clergy.
If the future pontiff decides, like SSPX, that the VII Popes were real Popes -- don't worry, this isn't going to happen, I'm just speaking hypothetically -- then our sedevacantist priests will probably not be given this after-the-fact jurisdiction. But even then, we have a safety net. This is that our priests have a "colored title." This means, roughly, that they are objectively out of the Church, but since we believe them to be in the Church, our subjective intentions are taken into account by God. In this case, they would be objectively but not subjectively schismatic. Since we believe them to be in the true Church, we can have hope that our confessions are not in vain.
During the Great Western Schism, there were rival Popes, but only one of them was objectively the real Pope, and only his clergy objectively were the real clergy. But that doesn't mean that all those who followed the false, objectively schismatic Avignon line of Popes are damned. Their priests and bishops would have had colored titles. Someone confessing to them would have believed they were confessing to a Catholic priest; they had no intention to reject the Church, or to be schismatic. So they are really inside in the eyes of God, one hopes.
The jureur priests of the French Revolution, the Revolutionary priests, were objectively schismatic. But they had colored titles, and those who confessed to them were told by the Vatican that they did not need to re-confess.
Since, on the basis of his having taught NFP, I don't think Pius XII was a Pope after 1951, the question of finding a bishop consecrated by him really means nothing to me, and the Siri thesis means nothing to me. Considering you have to be, what, thirty years old or so to be a bishop, and even that would be rare, that means the youngest bishop with ordinary jurisdiction out there would be in his early 80s. I know of no sedevacantist bishops who qualify.
Even if you accept that he was Pope until 1958, the youngest bishops with ordinary jurisdiction would be in their early-to-mid 70s, and we probably would have heard of someone like this by now, if he had become sedevacantist. There are those who accept John XXIII as a true Pope and if so, you have gained yourself another couple of years.
Not only that, but if a Pius XII bishop broke away from the VII Church, he might have ordinary jurisdiction, but those he ordained would not. This is the case with Abp. Thuc who actually was made bishop by Pius XI and had ordinary jurisdiction, but that does not carry over to those Abp. Thuc consecrated. Likewise, Abp. Lefebvre had ordinary jurisdiction, but the four bishops he consecrated did not. Jurisdiction isn't passed on like genetics. It has to be individually bestowed on each bishop by the Pope. So don't worry about finding a bishop that has it, because if you do, it will be by a miracle of God.
Ladislaus, do you or anyone else know the name of the late encyclical or address of Pius XII where he "decreed" that even if all the Cardinals are heretics they can still elect a valid Pope? If you accept Pius XII as Pope, this will create big problems for you. For me it is just another strike against him, showing that even during his pontificate, he was already establishing bulwarks to protect the future usurpers.