I think that the question goes hand in hand with the Church's infallibility, i.e. that one can never endanger one's faith or one's soul by remaining in a state of submission to the Holy See. All pre-V2 theologians would have considered the Traditional sedeplenist position to be schismatic.
It's always granted, however, that one would not be in formal schism if the lack of submission is due to doubts or questions about a Pope's legitimacy (they usually cite the Great Schism).
As I said, I have no problem in taking a position of doubt, i.e. we're not in submission because we have doubts about who these people are, but we'll stay sedeplenist because we don't feel that we have the authority to declare the Holy See vacant. That's really in line with Archbishop Lefebvre's own thinking.
But the radical Traditional sedeplenists IMO are formally schismatic. I don't see any way around this when looked at in the light of Traditional Catholic theology.
If you look at Bishop Fellay's Letter to the Three Bishops, he actually articulates these same principles ... and his reasons are logically quite valid. Bishop Fellay cited the principle that the Holy Spirit guides the Church and therefore starts to lean (based on his sedeplenist premise) towards the direction of saying that there's no substantial error in Vatican II, etc.
In that case, Bishop Fellay should have submitted unconditionally.
On the resistance side, however, the position ignores these considerations regarding infallibility and indefectibility and the Holy Spirit's guidance of the Church.
So, in other words, neither side is internally consistent.
Only doubts about legitimacy can justify the Traditional movement. Period. If you have no doubts, then you must submit to Rome and trust in God. If you cannot submit to Rome, then you must at least entertain doubts about the papal legitimacy to avoid formal schism.
So, in the final analysis, both the Bishop Fellay position (refusing to submit unconditionally despite granting that there's no substantial error in V2) and the Resistance position (refusing submission while maintaining a radical sedeplenism) are formally schismatic.
I saw the letter of the one French resistance priest, however, that seemed to open the door on sedevacantism as a valid opinion.
Both the SSPX and SSPX SO (aka Resistance) positions due grave damage to Traditional Catholic ecclesiology.
On a side note, I also think that radical sedeplenism also does harm to Traditional Catholic ecclesiology and can be schismatic.
I think that the only Catholic position is the sede-doubtist position, as I refer to it tongue-in-cheek. I believe that Archbishop Lefebvre and Bishop Tissier held this position. Some sedevacantists also hold it as just mere private opinion. But the radical sedeplenists and radical sedevacantists IMO are BOTH just plain wrong.