Sedevacantism is only secondarily about the pope at all (whether interior dispositions or heretical actions). It's about what we believe about the Catholic Church.
R&R: Church is capable of teaching grave error, of leading souls to hell, and of promulgating a Rite of Mass (Church's publish worship) that undermines faith and offends God.
SV: Church can't do that.
THIS is what the SV vs. R&R conflict is about, and often it descends down into the weeds where people lose sight of the big picture.
Conservative Novus Ordo agree with SV, while liberal Novus Ordo agree with R&R on this MAJOR (of the entire crisis).
MAJOR: Catholic Church cannot do such evil (SVs + Conservative NO) - rejected by R&R and Liberal NO
MINOR: Conciliar Church did such evils. (SVs + R&R) -- rejected by Conservative NO + Liberal NO
CONCLUSION: Conciliar Church is not the Catholic Church (SVs)
*Liberal NOs like the reforms, but might think, for example, that the Church did evil in the past
I grant that SV's think this is the logical conclusion of R and R but I'm not sure its what R and R usually/always says.
I guess the difference seems to be that in the R and R scheme its possible for a pope to take non infallible magisterial action WITHOUT that action representing the Church. Along with this sometimes would come the whole notion that the same Pope is the head of two different societies, the Catholic and the Conciliar, with all the non infallible heresy only being directed at the former.
I'm not saying it isn't a stretch, just trying to be fair to it.
You need more premises to make your point. I think the argument might look something like
1: If a true Pope, even in a non infallible capacity, attempts to bind Catholics to a heretical teaching, this destroys the indefectibility of the Catholic Church
2: The Catholic Church is Indefectible
3: The Conciliar Popes have, at least in a non infallible capacity, attempted to bind Catholics to a heretical teaching.
4: The Conciliar Popes cannot be true popes
So I think you argue as if R and R denies 2, but I think what they actually deny is 1, which you think logically leads to 2 (which might be true, but isn't the same as denying 2.)