I just want to point out one thing about the "Paul VI double" theory, that Bp. des Lauriers expresses support for in that letter. To us here in the 21st century it sounds certifiably insane, and objectively yes, I'd say it is, but at the time it was being discussed, in the 60s and 70s, the people that adhered to it did so for reasons that were actually pretty reasonable.
Here's how it's been explained to me. They were faced with a seemingly insoluble problem in the 60s and 70s -- someone apparently the pope, teaching heresy. This had never been seen in 2,000 years of Church history. So they couldn't figure out how to explain it without violating Catholic theology, except for the idea that Paul VI wasn't a free agent, or wasn't even himself at all, but had been kidnapped and replaced with a double. They would probably have agreed that the idea sounded crazy by itself, but would have said that it would be even crazier to assert that a real pope could be responsible for the changes of Vatican II.
So I don't consider the people who thought this (which included Bp. Thuc, actually) to have been completely nuts, although they are regarded that way today. They just hadn't heard the concept of sedevacantism, maybe, and it certainly took some time for people to research what all the theologians have said about the papacy and heresy.
All that being said, I do think it would go against the indefectibility of the Church for a pope to be replaced by an imposter, since if it were possible, we wouldn't be able to know if it had happened or not, which would call every act of every pope into doubt. Theoretically it would be possible that every pope since St. Linus had been replaced by a double, and it could be claimed that it was only at Vatican II that the true popes broke free from this bondage and began preaching the true gospel of Christ, which had been Bergoglianism all along. Obviously this is absurd.