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So there is a subjective element to it ... similar to what would have been the case during the Great Western schism.  If I was certain that my guy was the pope, I probably shouldn't attend a Mass una-cuм one of (what I believed to be) the Anti-popes.  But if I was uncertain, not sure about, which one was the actual Pope, I don't see that it would be a sin to attend any of the Masses for a decent reason.

Those citations about dropping the name being schismatic assume that the person recognizes that the man is the pope, but refuses to name him; that would constitute a schismatic act.

I hold that we're in a time analogous to that of the Great Western schism, where there's less-than-absolute certainty regarding the identity of the Pope.  Let's not get into the Universal Acceptance issue here, since we've had many threads on the subject.

Assume, for the sake or argument, that there's a serious doubt about the identity of the true pope, under those circuмstances, would it be wrong to assist at either una-cuм or non-una-cuм Masses?  I don't believe so.  I think you could go either way.  Even if you're a sedeprivationist, you could put the name in there given that he holds the office of the papacy, even if he lacks the formal authority on account of heresy, etc.

Given this situation, if I were a priest, I would keep the phrase "una cuм famulo tuo papa nostro" but then not insert the name, as a profession of formal intention to be in Communion with the papacy, even if there's doubt about who that is at the moment.

So there is a subjective element to it ... similar to what would have been the case during the Great Western schism.  If I was certain that my guy was the pope, I probably shouldn't attend a Mass una-cuм one of (what I believed to be) the Anti-popes.  But if I was uncertain, not sure about, which one was the actual Pope, I don't see that it would be a sin to attend any of the Masses for a decent reason.

I might even go a step further.  Applying the principles of probabilism (or semi-probabilism) [St. Alphonsus was first a probabilist and then after his position was criticized, he modified it somewhat], it would likely be morally licit to attend the Mass of any of these popes that you thought could possibly be the Pope.  In other words, if you considered it possible, even remotely possible, that one of these other guys was the pope, you could licitly assist at their Masses.  "Probable" in Latin didn't mean, like the current English, that "it's most likely to be true", but rather that something was "possibly true".  That's why in that one passage on explicit faith, St. Alphonsus referred to one position being MORE "probable" than the other.