But a papa dubius situation permits one (perhaps even obliges one) to not obey.
Correct. Papa Dubius Nullus Papa, so one can and, in the case where his directives go against Divine Law or Catholic Doctrine, MUST disobey.
But ... the thinking would be "in case I'm wrong, we will obey him where we can, since ... what harm does it do?"
That is the reason why +Lefebvre's position wasn't fatally defective. He made an unfortunately detour in the early 1980s when he was hopeful about Wojtyla (after Montini), where he made a lot of the same types of moves that +Fellay is making these days, but outside of that time period, he clearly was of the "Doubt & Resist" mindset, and not "Recognize & Resist", with recognize meaning to affirm. Even with affirming their legitimacy, there are degrees of affirming, and he never did affirm as if it were a dogmatic fact, and therefore certain with the certainty of faith.
Unfortunately, he did not emphasize these distinctions sufficiently, and so many of his later followers did morph into almost Dogmatic Recognizers.