I agree with what you said about the devil's limited angelic intellect and lack of full knowledge, but I disagree with your conclusion; for the simple reason that God often, for the further shame and humiliation of the enemies of the Truth, tells them in a general way (not in a specific way, with more details, like some of the inspired prophesies in Scripture) what will happen. The devils know Mary is going to crush them in the future, as God showed them long ago. And thus it is not unlikely God could have told them the SSPX will play a major part in Restoration.
"it is not unlikely". What, are you trying to sound more intelligent by using complicated language? How about you cut to the chase, cancel out the double negatives, and say "it is likely". (I guess you've been hanging around +Fellay and the Conciliar Church too long.)
But I say it's not
likely. The devil and God don't share a water cooler or break room, where God shares his intimate plans with the Adversary of Mankind.
So we're back to judging the SSPX on its current behavior and position: how well does it fight for Tradition, condemn the super-heresy of Vatican II, etc.
That is more reliable than blindly trusting the devil's testimony that the SSPX will triumph in the end.
Oh, and some food for thought -- what if Econe DOES eventually triumph? But it happens after the SSPX is purged of its liberal, pro-Modern World, pro-Vatican II, pro-Conciliar Church priests and bishops, and thanks to the steadfastness of the Resistance ends up helping to save the Church?
The Resistance would still be in the right, and the (currently unconverted, pre-conversion) SSPX would be in the wrong.
And actually, did the devil even say "The SSPX" or "The SSPX after their practical agreement with Rome", or "The SSPX under Fr. Pagliarani"? He said ECONE. That could be interpreted as the Resistance OR the SSPX. +Williamson is the spiritual offspring of Econe as well. Econe might mean +Lefebvre's actions to save the Church in the 70's -- leading to the Resistance, which eventually saves the Church.
So even that dubious prophecy says nothing about the current sellout-SSPX.