I just wanted to chime in. Yes, Xavier, I think sedes are certainly Catholic. (To pique at the sedes I also think Feeneyites are certainly Catholic). For the most part I mean as I do not know them all. My good friend from Church is named Tom and he is a sede and he always argues after our SSPX Mass about how the SSPX is weak and does not condemn Bergoglio enough. I am pretty sure I love Tom more than every other person in the chapel. Everyone else either hates him or merely tolerates him and makes fun of him when he is not around. His nickname is "Mount Aetna" because he explodes like a volcano when he argues about the Church. "The homo hierarchy". I think he is a good man. Pray for him as he has cancer. But to say my own position, I am an SSPX Catholic who is sympathetic to the resistance and loves Bishop Williamson, but the resistance chapel is far away.
I do not think you speak fairly about "the Nine" and their letter, Xavier. Though I do not know the details about the lawsuit and how they took those properties from the SSPX, their letter made some good points as I remember it. I have not read it in years, but I believe they questioned Novus Ordo annulments (a point which I agree with) and the Bugnini Holy Week (which I sympathize with), and the acceptance of Novus Ordo ordained priests (which I agree with). So I don't think the complaints were beyond the pale. I see sedes as allies and not schismatics, though I know a lot of people see everyone who does not agree with them on every point to be enemies. I would rather receive Communion from a CMRI priest or an SSPV priest than a Novus Ordo priest (even an indult priest) and I have never been to a sedevacantist chapel.