If a SSPX priest opted to go Resistance and decided to bring a civil suit against Bishop Fellay, would he be excommunicated from the Catholic Church?
If the Old Code of Canon Law remains in force, then it seems he would be excommunicated. I don't see how the new "Code of Canon Law 1983" (and is there a subsequent one?) can be valid since it's written by heretics with no true interest in preserving Catholic justice and order, therefore, it can't truly be in effect. But that's only my opinion. Only God knows for certain.
It seems that the material-only bishops & pope, who are not formal bishops & pope, can't truly impose a new Code of Canon Law, and the faithful are no obliged to follow it, just as we are not obliged to follow their obvious errors on matters of the Faith and morality in this modern/post-modern time.