The 1977 Certificate of Consecration shows Archbishop Marcel Lefebvre as Titular Archbishop of Synnada in Phrygia, a titular see that, while he never governed it, formally confirms his episcopal rank. As a validly consecrated bishop, he retained the full sacramental character and canonical protections of the episcopate. Under the 1917 Code of Canon Law (Canon 2318 §1), any cleric who publicly attacks, incites attack against, or sues a bishop incurs latae sententiae excommunication reserved to the Apostolic See, and this applies regardless of whether the bishop’s title is residential, titular, or emeritus. Therefore, both the historical evidence and canon law demonstrate that Lefebvre’s episcopal status was fully intact, and no claim that a lack of a titular see could exempt The Nine from canonical penalty is valid.
So, a blend of correct and incorrect here. 2318 refers to the Index. You must be referring to 2341. 2341 refers not to attacks or inciting attacks, but specifically to taking a Church official to secular court. So, apart from suing a very high ranking official, the section that's relevant is where you sue a bishop (even if merely Titular), and the penalty for a cleric is excommunication reserved to the Holy See "simpliciter" (the nuances of which vs. "speciali modo" aren't that important for our purposes). But then you throw in the "emeritus" incorrectly. There's no mention of "emeritus" incurring the excommunication in the 1917 code because it was in fact a Conciliar innovation, and since it was invented by Montini and The Nine did not recognize him as pope, and the argument is in fact, as per +Williamson, related to the "only Code of Canon Law they recognize" ... this renders any Montinian innovations entirely moot. At that point, you'd also have to assess their status according to the 1983 Code of Canon Law. So if "emeritus" is valid, then so is the 1983 Code, by which they are not excommunicated for these actions. BUT ... judged in the light of both 1917 Code still being valid and the Conciliar papal claimants being illegitimate, +Lefebvre retained his titular title, and therefore the excommunication reserved to the Holy See would in fact have applied to their actions. You are completely wrong that a lack of a Titular See would not exempt from the penalty, since it's stated quite explicitly in the Canon. It's just that a claim that he lacked a Titular See doesn't fly. Prior to Vatican II, every auxiliar bishop received one, and then once a bishop resigned from his Diocese, he took received one. +Lefebvre was granted one because the Superior General of the Holy Ghost Fathers should has a certain amount jurisdiction similar to that of a personal prelature, but was not restricted to a locality per se. Now, Montini could very well have stripped him of a Titular Title in 1976 when he suspended him, though there's no record of his having explicitly done so ... but, once again, in the paradigm of The Nine, anything Montini did was moot anyway.
IF in fact the 1917 Code of Canon Law remains in force and the Conciliar papal claimants have been illegitimate Anti-Popes, +Lefebvre retained his Titular See until death, and they did in fact incur excommunication reserved to the Holy See by taking the Archbishop to court.
Now, here's the tricky part for them. Even IF the Conciliar papal claimants were legit, as was the 1983 Code, even though Wojtyla promulgated the 1983 Code in January 1983, he explicited stated that the 1917 Code remained in force until the 1983 Code took effect in late November of 1983. But the legal proceedings by The Nine were initiated by the Summer of 1983. So, Bishop Williamson was a bit off, since whether or not other people recognize, that Code is moot regardless, since it had not taken effect until November 27, 1983.
Now, the one final condition condition in that Canon had to do with it only applying to matters related to the exercise of their office. This piece must be researched, and I don't know the answer, but if interpreted strictly, establishing churches in the United States fell outside the specific duties of a Titular Archbishop of Phrygia, but then that expression could and most likely does, IMO, refer to any ecclesiastical activity ... as opposed to entirely personal matter. So, for instance, if Archbishop Lefebvre had been driving a car and crashed into a cleric's car, the cleric would not incur the penalty if he took him too court to pay for damages to his vehicle, even though that would still be highly unbecomming and contrary to the spirit of the law. But setting up chapels, missions, schools in the US did in fact pertain generally to ecclesiastical, rather than to personal, matters, and therefore would fall under the penalty of the Canon, especially if you believe that his duties as bishop required him to take these actions given a vacancy of the Holy See.
So the window for an attempt to claim they were not excommunicated has shrunk to requiring a loose interpretation of the one qualifier in the Canon Law regarding the matters having to be related to the exercise of his office.