You say, " [m]any Masses were said una cuм the Papal claimants, and nowhere did any theologian come out and question the moral nature of assisting at those Masses." Are you saying that the people that assisted at those Masses believed the "pope" mentioned in the "una cuм" was an anti-pope? I think not. So it's a different situation.
Yes, I agree with you regarding the lack of a precedent. Which would indicate why there is "no concrete backing" in the historical record or even theological speculation . . . it's unprecedented, like the crisis we're witnessing.
Yeah, no kidding it is a different situation, I'm not unaware of that.
Now, since it is unprecedented, how is it that some who hold the non una cuм position can impose sin upon those who do not without a pronouncement of the Church? They cannot. It resides purely in the realm of theological opinion until the Church declares otherwise. Last I checked, neither +Sanborn, nor +Dolan, nor +Cekada, nor Novus Ordo Watch, possess the authority to do so.
Just like R&R, or sedeprivatonism, or sedevacantism; all of these being theological opinions on how to address this Crisis. Which is why not only the schismatic attitude ALL of us trads have towards EACH OTHER needs to stop but why pithy, worthless feuds like the one now starting between +Sanborn and +Dolan need to stop.