Technically, Bishop Williamson is their leader, but Fr. Pfeiffer is their "leading priest", it would seem.

The SSPX-SO was a name given to them by Traditio, I think. That name, so far as I know, is not official. The Resistance seems to be their official name.

The long term name will have to come from the Fathers, themselves. Earlier on, The SSPX of the Strict Observance appellation was applied, as was the " , Vienna Five", for the priests who authored the declaration from Father Ringrose's Church in Vienna, Virginia.

 I agree, seems the SSPX Resistance tells it all. Don't know about a formal election-- was really an informal appointment,I think. Father Pfeiffer, because of his abundant energy, homestead, knowledge of many of the coordinators and locales, and the humility of the others, seemed to be the logical informal choice for the near term.  Don't think Bp. Williamson  is  really the leader-- he's making himself available to
any resistance groups in all the various countries.

"strict observance" was a joke from a sermon.  It's generally good to avoid traditio-speak.

https://www.facebook.com/sspx.resistance

Archbishop Lefebvre was a very good man, but he was also a diplomat.  Furthermore, he did not understand exactly what was happening, though he clearly had more insight into the situation than almost anyone else.  This is completely understandable.  He had trust, up to a point, in the man he considered to be the pope.  We wanted to believe that Paul 6 and later John Paul 2 really did have the faith and really did have the best interests of the Church in mind during their pontificates.  Because of these realities, he vacillated.  He tried to work with the Conciliar authorities and spoke well of them until he found that he was betrayed and then it would start again.  He was confident at one time that the new ordination/consecration rites were invalid or, at least, doubtful but later decided they were acceptable.  He allowed open sedevacantists in his seminary for a time but later decided this was unacceptable and still later noted that he could not dismiss the possibility that the pope is not the pope.  It's all very confusing.

Today, all of the various factions of SSPX priests claim to be faithful to the vision of Archbishop Lefebvre.  The problem is that each group is correct for they are faithful to his vision of the Church at a specific point in time.  Bishop Fellay can be correct about making deals with the Modernists because Archbishop Lefebvre did, after all, sign a protocol with Rome even though he "unsigned" immediately after.  It all depends upon which Archbishop Lefebvre one claims to be faith to.

I think the various factions of the SSPX should place a date behind the initials (rather additional initials) to indicate just which vision of Archbishop Lefebvre that particular faction follows.