I agree indefectibility is a serious consideration that needs to be taken into account by all sides, but where sedevacantists err in this matter is in trying to use indefectibility in reverse, what you have justly syled modus tollens sedevacantism. Now, indefectibility was never meant to be applied in this manner and even the attempt to do so is heterodox.
I agree that one cannot use indefectibility for
modus tollens sedevacantism. I believe, however, that it can be used to establish positive doubt regarding the legitimacy of these popes.
Everything since the Council only requires the obsequium religiosum, and the Society renders this submission to the Roman authorities. The SSPX is not outside the communion of the Church.
I actually agree with you, AGAINST the SVs, that nothing clearly requires more than the
obseqium religiosum since Vatican II. But it just doesn't seem as if you're addressing Father Fenton's point. At no point can Magisterium get so corrupted that Catholics must REFUSE submission to the Holy See and to the Magisterium because doing otherwise would cause harm to souls; in other words, nothing from the authentic Magisterium of the Holy See (in particular when it's addressed to the Universal Church) could EVER require the breaking of submission to the Holy See.
You have redefined
obsequium religiosum (OR) here into the false lip-service "submission" that SSPX practices. That's NOT what OR means.
(2) The right to critique the Council on points where it is shrouded in ambiguity or imprecision, which lead to error in practice, with Tradition and prior Magisterial teaching being the judge and criterion.
OR entitles Catholics to respectfully question of the Magisterium through the appropriate channels.
Lumen Gentium itself in fact reaffirms this, and this is exactly where +Fellay is going. Where the Vatican has issues is in the open chest-thumping kind of rebellion ("We reject Vatican II.")
If you are having doubts or scruples about whether the Society and traditional Catholics who support it are in "full communion" with Rome,
There are no doubts or scruples involved, Nishant. SSPX is in open rebellion to and clearly rejects the V2 Magisterium. They conver the Sacraments, including those which require the power of the keys, i.e. jurisdiction, despite not having the jurisdiction to do so. They offer Mass in the diocese of local ordinaries contrary to their wishes. There's NOTHING that would qualify as canonical submission to the Holy See in the SSPX apostolate.
I'll comment more later, but you did not directly address Monsignor Fenton's post head on.