At what point can one say, without a doubt, fr. pfeiffer and fr. Hewko are heretics?
Formal heretics? Never. None of us can convene and head up an ecclesiastical tribunal to excommunicate anyone. We'd have to have authority over the priest(s) in question.
Now material heretic is another story. Any obvious deviation from Catholic doctrine makes one a material heretic. But in this category would go all those who don't know their Faith well enough to make no mistakes in theology. In other words, a person can accidentally stumble upon material heresy. For example, I've heard a pious lady call God "the greatest angel of them all" -- but saying God is an angel, a separated substance, a created being, is pure heresy. But she obviously didn't mean it like that. She meant the greatest SPIRIT or something like that.
But to prove obstinacy in a given heresy, any cleric has a right to an ecclesiastical trial, as Martin Luther was given. Only after this can a person be excommunicated by the Church.
Only the Dimond Brothers and other schismatics attempt to be a "lay pope" and excommunicate people left and right, declaring them vitandus
(to-be-avoided) at the same time.