"For example, if a sedevacantist states that Novus Ordo masses are invalid because they are Novus Ordo, he crosses over from sedevacantism, which question apostolicity, into a general schism with Rome on ecclesiological grounds."
As with Protestant ecclesiology, there are inherent problems with the SV position, given the papist position of the SVers. I personally think this can only be resolved by SVers going to either an Old Catholic position or to Orthodoxy. As Old Catholicism is imbued with a heavy dose of Jansenism, however, I don't see SVers going there. And if they go to Orthodoxy, they'll have to dump a lot of 19th century RC piety. In other words, the SV position, as it is now, is not tenable. They have to go somewhere and when they do, they'll have to dump something. My guess is they'll go wherever they don't have to dump as much. Off hand, I can think of two Orthodox synods they might fit into.
Or maybe they'll become the Western equivalent of the Russian Old Believers?
Educate me. What is the Old Believers theology? I know about the two fingers, not much beyond that.