Thank you ... The reason being that the reasons for the invalidity of his claim to the papacy have no bearing on the problem of the long-term vacancy of the Holy See, which is a separate issue (though certainly a fair one to bring up).
Sure. Let's talk solely about Pope Benedict and some of the problematic statements he has made. Here's the issue, although it seems evident that they contain material error, I would argue they are not sufficient to condemn him as a formal heretic.
Here is how the SSPX website puts it,
But nobody can authoritatively admonish the pope (canon 1556), and the bishops can only be admonished by their superior, the pope (canon 1557), who has not done so.
* To have canonical force, they must come from one's superior (cf., canon 2233). The point is not only the crime but also its imputability must be notorious (canon 2195; 2197).
Therefore, pertinacity, and so formal heresy, cannot be proven.
In other words, pertinacity is generally determined by a person refusing the reproval of his ecclesiastical superiors. But the Pope has no such superiors in the constitution of the Church? So, what is the solution in this case?
Some think a Council called by the College of Cardinals as Cupertino somewhat said, citing some approved source, would have to publicly rebuke him for an error and then demand he recant. And only his obstinacy in the face of such an admonition could suffice to canonically establish pertinacity necessary for him to be a formal heretic.
But if this is true, one could argue that it is impossible that Cardinals would ever cease to exist, as the 50-year-sedevacantist position maintains they have, and therefore that position must be in error.
So then, what are we to do without a Pope? As you said, where does it end? I have two things I would like to mention in response. First, Mr. John Lane put it very well when he said:
"It is no solution to treat heretics as legitimate pastors of the faithful - that merely compounds the disorder. It is also a failure to act in accordance with truth, in favour of an imagined legal fiction." (quoted at http://www.novusordowatch.org/the_chair_is_still_empty.htm)
For the reasons I mentioned, it is slightly more complicated and uncertain than that. The examples of Pope Liberius and Honorius and Sts. Athanasius and Maximus respectively also give credence to the SSPX view. What was the most important thing is to keep the Faith and proclaim the truth, not to jump to what may be a very rash conclusion, or at least an insufficiently established one.
By the way I think someone cited Felix II, he was an Antipope, and Pope Liberius was wrongly declared deposed. Just an example of jumping to conclusions and the attendant dangers it involves. Much better to stick to what is absolutely certain, as did St.Athanasius.
Who will decide when the Pope is once again "okay" and need no longer be resisted?
When the issue at hand is resolved. It really revolves around ecuмenism, collegiality and religious liberty, these are the disputed points, and because they have not been sufficiently clarified, the SSPX response is justifiable.
Both SSPX and FSSP await a clarification from Rome, but the latter prefers to work with the Magisterium where possible, remembering former examples like slavery and usury where the Magisterium had appeared to change its position on complex issues while actually expounding a consistent position in changing circuмstances, though this was not immediately clear to all. As with Pope Liberius, any difficulties that originated in Rome will be resolved from Rome, for the gates of hell cannot prevail against the Roman Church built on the Rock of St.Peter.
Who will decide when the Magisterium is back to its senses (as though it could ever even lose them)? Who gets to say when canonizations must be fully accepted, encyclicals can no longer be criticized, and canon law can no longer be called wicked?
I admit there are certain points of internal discordance or inconsistency in such a view, which is why I don't entirely embrace it. There would be about 2% difference between the SSPX and the FSSP and this is one of it.
In short, when will the SSPX know to revert to a pre-1962 position
When the three issues I mentioned are satisfactorily and visibly reconciled to Tradition. In the meanwhile, if it is not clear how they are to be reconciled, one can hold to Tradition as such, because Vatican II specifically said it neither taught nor bound anything new on anyone.
If the SSPX is right, then any group can always "resist", for whatever reason seems "traditional" to it, and do its "own thing."
That's an oversimplification, I believe. The Church's traditional doctrine on these subjects is well known to theologians, and cannot be actually changed by anyone, no, not even a Pope, of course, and there can only be development "in its own proper kind, that is to say, in the same doctrine, the same sense, and the same understanding" according to the First Vatican Council. The onus is now on Rome to show precisely this, and I believe it will come.