Thanks for your insights. You addressed what troubled me in the article about heresies, and I thank you.
This is inaccurate. Automatic excommunication has a vastly higher standard than hearing a Mass said by a priest said to be schismatic. Automatic excommunication is for defiance of well-known Church teaching on a serious matter such as abortion. Abortionists, those who deliberately and knowingly participate in abortion, and abortion-supporting politicians excommunicate themselves by their actions.
This isn't to say it's a good idea to go to Mass at Pius XII chapels. What I have read is that such a Mass is canonically described as "valid" (it truly confects a sacrament and fulfills your obligation) but not "licit" (those priests are not supposed to be confecting the sacrament under those circumstances). But in a sense, it's not your problem if you're just attending, because the standard of accountability for laymen is lower. The Society being declared schismatic doesn't make you schismatic yourself, because to be a heretic or schismatic, you have to be knowingly, deliberately, and publicly defying the Church's line of succession from Christ and declaring your allegiance to a different hierarchy. Just the requirement of "knowingly" cuts out most members of the laityincluding most of Luther's followers!
As some have commented, the problem with SSPX isn't so much the priestsit's the congregation! The priests at least publicly recognize and pray for the Pope. But anecdotally, in the U.S., the parishioners typically include a big proportion of those who talk as if they are sedevacantistsfolks who say the Pope isn't the real Pope. I don't think it's a great atmosphere for one's spiritual confidence and development.
To draw an important distinction, my understanding is that Masses said by sedevacantist priests are not valid, and do not fulfill one's obligation. But to hear even such a Mass would not cause automatic excommunication for the laymen attending.