In my experience with sedevacantists (not on this board or online), I find that there are a good number who believe in Our Lady of Fatima's apparition and pray to her, but believe that her demand for the Pope to consecrate Russia and her promise of an era of peace is of no relevance, as if Our Lady was lying or God changed His mind about how He wanted to do things. Probably because "there is no Pope" to consecrate Russia. I think if there was going to be no Pope around to do the consecration or there was at least no possibility of there being another Pope to come, then I dont think Our Lady would have bothered asking. She knows good and well what's going to happen, because she doesnt do anything outside the will of her Son.
So yeah, most sedes believe in Fatima. I think question is, do they believe ALL of it?
Btw, I dont mean to contradict your individual experiences regarding sedes in this matter, Im just contributing my own experience and observation. My bunch was/is wacko.