1) The Nine were not expelled for holding the sedevacantist position. As a matter of fact, Father Collins was not a “sede” at the time.
2) Infallibility has nothing to do with it. If one is a manifest heretic he ceases, by that fact, being a member of the Church whether he be a layman, a priest, or a pope.
1) Not so, ask any SSPX priest who was around in those days - and +Sanborn was the ring leader. Every old time SSPX priest I ever spoke to about it all said the same thing.
2) As I said: sedes and the pope both believe it to be a teaching of the Church that whatever he says or preaches in union with the bishops is infallible.
If you have faith in that teaching, then neither he nor the other conciliar popes can possibly be manifest heretics, instead, what sedes believe to be heresy, is in fact, without the possibility of error.
Either that, or sedes have no faith in that supposed teaching of the Church. There is no other option.