So far as Fr. Hesse not being sedevacantist:
1) Apparently, some would save him from that stigma because, though he rejects Francis, he thinks
someone else is Pope;
2) According to that rationale, the conclavists are not sedevacantists, because, though they reject Francis, they believe Michael, Gregory XVII, or Pius XIII (back in the day) are/were Pope(s).
If you will counter that, unlike the conclavists' "Popes," BXVI was elected by due authority, then I respond:
1) That he was forced out is unprovable speculation
2) And even if he was, you still run into that pesky quote from Billot (which once again, was/is the common opinion of approved theologians):
“Finally, whatever you still think about the possibility or impossibility of the aforementioned hypothesis [a Pope becoming a heretic], at least one point should be considered absolutely incontrovertible, and placed firmly above any doubt whatever: The adhesion of the universal Church will be always, in itself, an infallible sign of the legitimacy of a determined Pontiff, and therefore also of the existence of all the conditions required for legitimacy itself…As will become even more clear by what we shall say later, God can permit that at times a vacancy in the Apostolic See be prolonged for a long time. He can also permit that doubt arise about the legitimacy of this or that election. He cannot however permit the whole Church to accept as Pontiff him who is not so truly and legitimately.Therefore, from the moment in which the Pope is accepted by the Church and united to her as the head to the body, it is no longer permitted to raise doubts about a possible vice of election or a possible lack of any condition whatsoever necessary for legitimacy. For the aforementioned adhesion of the Church heals in the root all fault in the election and proves infallibly the existence of all the required conditions.”3