In the dialogue between SSPX and CDF, I do not see how SSPX can avoid being asked to face up to the question of the status of the post-conciliar papal Magisterium. I am afraid I suspect some of them of being constructively Sedevacantist. I know that SSPX has waged a relentless war against those who are formally sedevacantist. But I cannot help sometimes feeling that though they formally acknowledge the validity of the election, say, of JP2, and of his pontificate, some members of SSPX behave as though in fact his magisterial pronouncements lack full status and do not demand obsequium. He was for them, one might say, a merely nominal or titular pontiff.
For those with a sense of humour, an obvious solution presents itself. SSPX must accept the pronouncements of post-conciliar pontiffs ... with the same respect that liberals accord to those of pre-conciliar popes. And indeed, when some liberals demand that SSPX be made to adhere to 'every syllable' of the conciliar and post-conciliar Magisterium of Vatican II, it is right to check up that these gentlemen themselves adhere not only to every syllable in the documents of Vatican II but also to the totality of Vatican I and of Trent and of the post-Tridentine Magisterium. All that would most certainly have its funny side. But this is a serious matter. And canonical regularisation can hardly await the completion of scrutiny of every syllable in official documents since the 1960s.
I wonder if it might be felt that a regularised SSPX should be expected to show appropriate submission to future magisterial documents that emerge in a Church in the life of which they have been playing a full part; while, as far as previous magisterial enactments were concerned, they would demonstrate a polite respect which did not exclude the possibility, in theological discourse, of contextualising those documents. Raking up old problems just for the sake of it is not necessary in ecclesial life; although of course an old decision might need to be examined and reworked if the questions with which it dealt resurfaced disruptively in the life of the Church or in acrimonious dissent between theological schools. Here again, such an arrangement would look ahead to reconciliation with groups such as Byzantine Orthodoxy, where such an understanding would be unavoidable. A lot of water has flowed under the bridges of both Old and New Rome since 1054.
So, once more, I feel (and sometimes I wonder if this is what the Holy Father has in mind) that the SSPX/CDF dialogue could be a dry run for the Big One. Please God it may be so; and that the Spirit may rest upon those involved in this autumn's exchanges.