Speaking from the other side of the fence, as a non-sede who is nevertheless not 100% persuaded one way or the other --- and I ultimately settle the doubt, for practical purposes (such as this one) in favor of Francis being pope, though it's not exactly the high point of my day to have to accept him --- I have tried to justify assisting at non-una cuм Masses, but I just can't do it. I've gone to non-una cuм Masses before, using the reason of "a priest being in error as to the status of the Chair of Peter in the here and now, similar to a priest who gets up one morning, hasn't read or turned on the news yet, the Pope died in the middle of the night, and then he goes to the church and celebrates una cuм, not knowing that there is no Pope". That's pretty tendentious, and while it sounds good in theory, I can't convince myself that it would be a good general rule to follow.