1. Of course it's a novel idea (but not really because theologians discussed it) but not because 'regardless of who the ones are spreading error' but precisely because of the ones who are spreading the error. Novel ideas that aim to wreck the Church (Sillonism or Modernism, for example) are different than theological ideas which aim to preserve the papacy and indefectibility of the Church (Sedevacantism).
2. If it was a case of a pontiff making an error in some book that would be something but really that isn't what's being talked about. It's the universal enforcing (which he can do) of false / condemned principles, a new faith, new sacraments, new theology, new priesthood, new 'mass', new vestments, etc. (which he can't do because the Church is protected by the Holy Ghost and can't be an institution that teaches, promotes and enforces error).
1. It was never our job to preserve the papacy and the Church's indefectibility (Sedevacantism), but if it were, we must admit that it is altogether impossible to preserve anything by first eliminating the thing you're aiming to preserve.
2. See, here I will stick to what I said previously. Sure, popes can enforce error all they want, but the only way to hope to cure that, is to remain steadfast in the faith and not submit to those errors as trads have done. We all know that remaining steadfast in the faith is not novel, it has always been a condition necessary for our salvation.