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To answer the main question here, of how sedevacantists think the Church can get a pope again, sedevacantists generally agree that if Francis (Bergoglio) were to repent and publicly abjure his errors, he would become a valid pope. Different schools of thought explain this differently, but they pretty much all agree on the basic idea. The people who adhere to the Thesis of Bp. Guerard des Lauriers would simply say that he was already validly elected, and simply removed the obstacle that was preventing him from being pope, so he forthwith becomes pope. Sedevacantists who do not adhere to the Thesis of Bp. Guerard would probably say Francis became pope by being accepted by the whole Church as pope. This is called election by "acclamation", and I believe it has happened in the past. It has been discussed by St. Robert Bellarmine and numerous other theologians.
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Alternatively, most sedevacantists would probably accept it if several Novus Ordo cardinals -- or possibly even bishops -- renounced their errors, denounced Bergoglio as a modernist heretic, declared the papacy vacant, and elected a pope after he refused to recant his errors. Sedevacantists would accept a pope resulting from this process for similar reasons to the prior scenario I mentioned.
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This is off the top of my head so some sedes might want to chime in and tinker with some of the details of this, but substantially that's the answer to your question. Note that it doesn't place any time limit on the Church. And yes, there are objections that can be made to these ideas, but they are of trifling difficulty in comparison with the difficulty of saying that real popes have been behind the universal heresy and apostasy of the Vatican 2 religion.

Given what anonymous sedevacantists say here and there, yes, there are a few who say such things. But have you ever heard any named sedevacantist, whose name has been heard before here and there, utter such absurd ideas?

Repentant heretics who have actively been destroying the Church would want to spend the rest of their lifes in a dungeon and play no part in determining any successor. Who are such to ever open their mouth again?

You say now these men “appeared to be bishops of the Church,” circling back from their subsequent heresy. The day before Pope John XXIII convened the Second Vatican Council, he and all those bishops were universally recognized as the living Magisterium of the Catholic Church, which in fact they were.

Well, the Magisterium is only protected from error when the bishops are teaching in union with the Pope.  Ephesus II taught Christological error and was later repudiated by Pope St. Leo the Great as a Latrocinium.

All Trinitarian Dogmas remain the same. All Incarnational Dogmas remain the same. All Marian Dogmas remain the same. All Eucharistic Dogmas remain the same. Things like Ecuмenism are not Dogmas at all, but pastoral practices based on the possibility of good faith or invincible ignorance or material heresy among separated Christians. Once it is admitted separated Christians can be in good faith, and yet must still be reconciled to the Body of the Church, the supposed "heresy" in Vatican II disappears. There is no heresy in Vatican II. Nor can there be, for exactly the Reasons of Indefectibility mentioned by Pope Bl. Pius IX in Etsi Multa.

Papal appointment is not the same as Consecration so, even after the last Pius XII appointed bishop dies, the Apostolic Succesion will not be broken

Disagree. Apostolic Succession requires both Orders and Jurisdiction. One or the other by itself is not sufficient to maintain the succession. Consecration would transmit orders, but only Papal appointment will transmit jurisdiction. Therefore, that Papal appointment is necessary, and therefore the See cannot be vacant for 62 supposed years, as the sedes hold.

The same conclusion follows another way: the First Vatican Council says there will be Shepherds and Teachers in the Church until the end of time, who were sent just as the Apostles were sent. That is clearly a reference to Bishops who have power of teaching and of ruling, the Magisterial power and that of jurisdiction. The reference to "sent as the Apostles were sent" is another reference to canonical mission. These things are explained by theologians.

Msgr. Van Noort is one example: "What is required for genuine apostolic succession is that a man enjoy the complete powers (i.e., ordinary powers, not extraordinary) of an apostle. He must, then, in addition to the power of orders, possess also the power of jurisdiction. Jurisdiction means the power to teach and govern. - This power is conferred only by a legitimate authorization and, even though once received, can be lost again by being revoked. [Christ's Church, Monsignor G Van Noort]"

XavierSem, all these papally appointed bishops adhere to the heretical robber council. They lost their offices just like all the antipopes lost theirs

Struthio, if all these Bishops along with the Popes lost their offices and promulgated heresy, the Catholic Church then and there died and defected. It is impossible. The comparison with Ephesus II is not correct at all; that was a purely local Council, and from the first, as "Shameless Popery" quoted by Decem Rationis has docuмented, was rejected by the Popes. The claims of the sedevacantists here are like the claims of the Old Catholics vis-a-vis Vatican I. That was already answered by the Pope: to claim heresy in a Universal Council of Bishops like Vatican I - not a merely local Council only, like Ephesus II - denies indefectibility.

Jerm, agreed.

Yeti, as you can see, Struthio doesn't agree with your theory. Sedes will not be able to come to agreement about it. But if you go by what older Theologians wrote, Fr. Suarez says you need to gather the Ordinary Pastors of the Church in Council. These Theologians envisioned only one single Pope possibly (and then not in Council) falling into heresy (as a private person only), and then a Council being convened, within the lifetime of that Pope, of all Jurisdictional Bishops ("Ordinary Pastors") to determine his pertinacity or lack thereof, and declare him deposed if he continues obstinate. Most said this was only a hypothetical and would never happen. But if you wish to claim this happened to H.H. Pope John XXIII and all his Successors, you need to show "Ordinary Pastors" ready to declare it.