The approach in the Open Letter is actually quite interesting and has much to commend it. For one thing, the signatories explicitly avoid choosing and openly declare they have no intention of choosing one side between Cardinal Cajetan and Cardinal St. Robert Bellarmine - that, they say, is for the Church Herself i.e. the Bishops to determine and rule on how they wish to proceed. It's actually an eminently defensible canonical procedure, it leaves everything for the Bishops to judge. Unfortunately, I don't think many Bishops have stepped up.
There are some 5000+ Bishops in the Catholic Church. Supposing at least 50 Bishops, 1%, begin to say, something like, "The Pope has fallen into heresy. That is sure and clear. Now we're working out whether he is obstinate, or ready to correct and retract." At that point, we could consider whether the Pope was losing his office. Both the heresy and the pertinacity must be manifest, i.e. the Pope must be manifestly obstinate in defending his heresy, even knowing he contradicts the Church in doing so. At that time, no longer universally accepted, he would fall from office. And then the Bishops of the Church could declare this fact, and then with Cardinals elect a new Pope.
It's unlikely all this will go ahead right now - but at least one good thing has come out of all this, the heretics in the Vatican now know the Traditional and even Conservative world is watching and won't just let them get away with anything. I think the best thing to do would be for good Bishops and Cardinals, like in their recent declaration of Truths, to define a few important dogmatic Truths taught by Tradition, and say that anyone who teaches otherwise, by the Church's Traditional Teaching, is anathema. Or, they could all profess the anti-Modernist Oath, or some other traditional Creed, like the Tridentine profession of Faith, and demand the Pope profess it. Or so on. But they have to step up now, imho, otherwise the process can't proceed much, since by divine law, they are the only judges with habitual ordinary jurisdiction in the Church beside the Pope.