"Even in stating that I believe it is a fact that BXVI is a non-Catholic (and, a fortiori, a non-Pope)..."
This is a crucial part of the armory against those who say we're judging the "Pope." He is not a non-Pope before being a non-Catholic. It's the other way around. He is not a Catholic and that means he can't be Pope.
However, unless we are near the Second Coming, someone WILL have to do the unthinkable and judge these "Popes," deposing them legally so there can be a true Pope, with the Council of Constance as our model.
Some sedevacantists say "We have no legal right to depose anyone" but actually they do. The true bishops and priests who rejected VII can very well start calling themselves the true Catholic Church because that's what they are.
It's just that if we elected a Pope now, it would make us look like homegrown eccentrics like David Bawden. There has to be a substantial turnaround in the Catholic ranks before we can name a Pope. Many will have to wake up that the VII Church is an anti-Church.
This is why I say that the SSPX is harmful, because it's keeping traditionalists allied, however tenuously, with the "robber Church" as Patrick Henry Omlor calls it. This keeps the sedevacantists as a fringe minority, relegated to the shadows, and allows the Godzilla monster of VII to rampage unchecked.