I'm highlighting the problems with your Pope-Sifting Sedevacantism. If you think a Pope is a heretic, you just declare the See vacant. Problem solved. You therefore undermine the entire Magisterium. If a Pope were to come along who condemned BoD, would you accept it? Most likely no. You would declare the See vacant. Consequently, there is no Magisterium anymore other than the private judgment of Ambrose. For as much as you sanctimoniously aver your loyalty to (your interpretation of) the Magisterium, YOU ARE IN FACT YOUR OWN MAGISTERIUM.
There is no sifting, there is the realization of a public crime, namely heresy.
If you saw a man enter a bank with a black mask, gun drawn, who demands money, and then runs away, can you judge this man to be a bank robber, or must you wait for a judge to declare this fact?
If you witness a man claiming to be Pope teaching in an authoritative capacity, heresy and grave doctrinal errors, and promulgating laws contrary to the Catholic Faith, can you recognize this fact or must you wait for a future ruling before you can be aware of a plain obvious fact?
If you say the latter, then you are completely defenseless against this heretic, if you truly regard him as Pope.