It's Father Rioult who revealed the news I mentionned in my post above. Concerning Father Chazal, he is not sedeprivationnist. Here is what he wrote recently in one of his email (translated from French) :
One cannot say that the public life of the Church, the apostolic and visible continuity of the Church is something purely material. No, it is something formal, that is to say very real. Pope Francis is a suspect awaiting conviction and deposition.
So, to answer your questions:
1. No, I am not sedeprivationist.
2. The difference between Cassisiacum and Archbishop Lefebvre is that we are content to separate ourselves from the conciliar Church and leave to God the resolution of the question of loss of office.
He also condemned sedeprivationnism in his recent book (Contra Cekadam) against sedevacantism :
* Canon 160, against sedeprivationnism. "The election of a Sovereign Pontiff is guided solely by "Vacante Sede Apostolica" of Pope Pius X", which constitution, on #29 not only exclude canonical or juridical censures, but also any reason whatsoever to bar a Cardinal from active or passive voice in a Conclave. More on this later.
* Hence it is impossible to find any trace of your sedeprivationism in the legislation of the Church, and Cum Ex has fallen out of use, if it were ever used to bar a Cardinal to the Papacy. Fr GREGORY HESSE explained that Cum Ex was not used, save for its principle (that the holding of an office is incompatible with heresy), because of the regrettable tendency of Paul IV to imprison clerics without trial.
The last big problem of sedeprivationnism, is that Cardinals are no Popes, so, even in the theory of sedevacantism, they don’t enjoy immunity; they must be judged. We were told by them that special rules, dispensing from a juridical sentence, apply only for the Pope. With sedeprivationnism that is no longer the case, and therefore all ecclesiastical offices are in doubt the minute an individual Catholic deems the holder to be a heretic. Indeed, many sedes say that Cardinals are non Cardinals, bishops fake bishops, priests false priests. It is the proof that much more is at risk than the sole office of a Pope, but all offices in the Church.