I was using “fiat” in the generic sense of “I am regularizing this, because I am the Pope and I have the authority”. What form it would take, a motu proprio, or whatever, is beside the point — that part is just technicalities. The tricky part is getting “just as they are” in line with the bare minimum the Church requires (e.g., not denying any part of Vatican II, yet raising the issue that certain documents are difficult to understand in the light of tradition, and seeking clarification).All it would take is a fiat from the Holy Father to bring the SSPX into a personal ordinariate “just as they are”, thus regularizing the SSPX and everything they do
This seems to be more an issue of canon law than anything else. I do not know whether canon law addresses the morality of receiving sacraments from priests and bishops, who are not themselves schismatic or “outside the Church”, yet are administering sacraments illicitly. I would look to a canonist, or at the very least, a priest who is not hostile to the SSPX and does not regard them as schismatic (because they’re not), for further guidance.It seems, then, that conferral of the remaining five sacraments, while technically illicit…