The impression that I have is that after Abp. Lefebvre formally left the SSPX leadership (1984 I think) and specially after the 1988 consecrations, the ruling party (Fr. Schmidberger, Bp. Fellay and company) developed the "dialogue Papacy" theory, along with dogmatic anti-Sedevacantism and blind obedience to the superiors.
This allowed them to keep the group's unity, but at great price.
Abp. Lefebvre was clearly not anti-Sedevacantist, as it can be seen on his commentaries on the 1986 Assisi scandal. His position on the crisis was not the one that the SSPX adopted after his death, and I think that this change happened long before the "2012 shift".