Sorry, Jehanne, you are mistaken. I think sedevacantism has hardened you, since what you used to say on this matter was something else, which I thought was better, even when I disagreed.
For sin to be mortal, one must be culpable for it, otherwise there is no internal effect, whether that be loss of sanctifying grace or supernatural faith. This is true for all mortal sin, and for the mortal sin of heresy and schism in particular.
But though the doctrine which men hold be false and perverse, if they do not maintain it with passionate obstinacy, especially when they have not devised it by the rashness of their own presumption, but have accepted it from parents who had been misguided and had fallen into error, and if they are with anxiety seeking the truth, and are prepared to be set right when they have found it, such men are not to be counted heretics. Were it not that I believe you to be such, perhaps I would not write to you.
St. Thomas discourses in the same sense.