I've been repeatedly accused of just "making up" some notion that the Church's Magisterium is indefectible, but we clearly see Pope Pius IX teaching it here. Without an a priori dogmatic understanding that the teaching of the entire Church (meaning all the world's bishops teaching in union with the Pope) cannot err, the dogma of papal infallibility has nothing to stand on, and the Old Catholics MAY have been right. But, since Etsi Multa wasn't infallible, maybe he was wrong about that too, no?
So, we say that we have dogmatic certainty about solemn papal definitions because of the dogma of papal infallibility. But what about BEFORE infallibility was defined? Did Catholics before Vatican I have no dogmatic certainty about authoritative papal teaching? In fact, how can we be dogmatically (absolutely) certain that papal infallibility is true unless we're ALREADY dogmatically certain that the entire Church (Pope and Bishops) cannot err in teaching the entire Church? Answer is that without such an understanding of the indefectibility of the Church's Magisterium, we can't be, and it's not dogmatically certain that the Old Catholics were wrong. Heck, if an Ecuмenical Council could go off the rails as it did at Vatican II, what says Vatican II didn't already go off the rails before it?
There's really only two ways out of it, R&R:
1) Either accept that Vatican II was essentially Catholics (perhaps abused, misinterpreted, with some ambiguities that need to be properly resolved by the Church's authority.
2) Or two, assert that the teachings of Vatican II were not those of the Pope and the Bishops teaching in union with him, which would mean that Montini was not the Pope.
There's simply no other way to resolve this problem in a Catholic manner, except in one of the two answers above.