Sedevacantism is not the only doctrinally-sound conclusion. I am not a 1958 Sedevacantist. But I understand their convictions, and their conclusions follow logically from their convictions.
Agreed. The facts are unclear and those particulars of how, who, & when can vary to some degree.
The idea is, "One should follow their convictions based on the formation of their conscience according to Catholic teaching."
To ignore a well-formed conscience that is alerting one to these contradictions in the teachings is perilous for one's soul.
The chance that Stubborn already had his time to consider these things is probably well past.
His intransigence is the natural outcome of his pertinacity.
No human arguments can shake him from his stupor.
Or he could say that the errors were actual heresies, but the Popes were not to blame because the Cardinals were manipulating the situation and tying the hands of the Popes.
Maybe this could apply in some situations, but I do not think it theologically sound for what we are living through. It seems to cut against the freedom of Church.
And the reason I continue to discuss it is not only for Stubborn's benefit, but for all of the dogmatic R&R crowd on this forum who parrot the nonsense taught by the SSPX.
Sure, but at a certain point it is just endless "circle flying with no end in sight".
It doesn't take but one word to reignite the debate with someone else and Ol' Stubborn will be at it yet again.
This time it was you doing most of the arguing, last time it was me, the time before that it was Pax, next time it will be Lad, etc.
Come at it from the POV of membership, heresy, moral certainty, Old Catholic mentality, whatever - it is all quite pointless with him.
The day Stubborn accepts the teaching, that day - CathInfo will change it's name to SedeInfo

But, for the benefit of others, you are right - it must be gone over again, and again and...