“How would the priest witness the lack of absolution without himself, as a free-willed man, taking the decision not to absolve?”
That’s easy, Alex. He simply would not read the absolution prayers. The priest does not absolve; he is merely a witness and a conduit as the words of the Mystery make clear. If God is doing the absolving in the Latin Rite of the Mystery, why does the priest say “Absolvo te”? No other rite makes that presumption. Personally, I think it has to do with, among other things, a different concept of the meaning of priesthood as between the Latins on the one hand and the other particular churches within The Church on the other.
Because, just like an Orthodox priest, he made the decision to absolve, and just like Stavros' priest he could make the decision not to absolve if the penitent lacked proper disposition.