Thanks for the explanations, on both counts. That makes a lot of sense. From a different angle, another reason the SSPX using NO churches bothers me is that it gives the NO bishops a semblance of holding lawful authority in the Catholic Church, and how they are charitable, merciful in deigning to allow a "canonically irregular" group to celebrate the true Mass in their churches. Now obviously a Catholic bishop does have the right to do that, and I guess the sspx has always held those bishops to still have authority..but it just another one of those things that I think can cause trad faithful to become complacent, and believe the situation isn't as bad as it really is
Well, that's what they believe, though, that the NO bishops do have authority. So, for instance, St. Vincent Ferrer thought that one of the Antipopes was the actual pope and (presumably) put his name into the Canon and then the name of the bishops who was under that false pope, etc. He did so because he thought they had authority, but was wrong. That too I put in the same category, that of material error but with the correct formal intention.