Father Cekada makes a reference cannon 151 of the 1917 Code of Cannon law, but it's in Latin.
Canon 151
(1983 CIC249 154)
An office that is vacant by law but that perchance is still held by another illegitimately can be conferred provided that, duly according to the
sacred canons, this possession is declared not to be legitimate and that mention of this declaration is made in the letter of conferral.
My personal comment is that you have to listen to each argument and rebuttal of these theologians and check their sources to see who is right. Each side says the other is in mortal sin (against Sedes: mortal sins of disobedience to a heretical pope and Vatican 1 telling is that those who say the seat is empty are an anathema to the church. Against backing Pope Francis the mortal sin of disobeying church dogma, the actual faith of the Church.)
Following the faith of the church is disobedience to Pope Francis, thus mortal sin.
Following Pope Francis is heresy, thus a mortal sin.
It's a damned if you do, damned if you don't situation. Which raises the possibility that we have become a wicked people undeserving of the promises of Christ, thus all damned.