There are a handful of dogmatic non-una-cuм types out there, but most are not.

What's under discussion is the implication that +Lefebvre may have gone sedevacantist before he died.

False.  They don't have to prove anything. 

This isn't about whether (una cuм is) sinful.

Maybe "on paper" they aren't dogmatic about it.  But the stories I hear, directly from individuals who attend these chapels, is that this topic is talked about A LOT.  And many laity absorb these "talking points" and apply them in their lives, against una cuм Trads, with disasterous results.  Fr Jenkins' chapels included. 

Dear Stubborn,

The names of a pope or a bishop in the Canon of the Mass are just like the feast days of saints; movable (people die, some become new saints).
Canon XIII of the Seventh Session does not preclude the changes in the liturgy outside of the rites of the sacraments. One does not incur anathema by not putting the name of the pope or the bishop into the provided space. These men even in regular times can be out of Office ( the state of sedevacante). One does, however, incur anathema by putting the name of St. Joseph to an unchangeable Canon of the Mass. Pope John XXIII is under anathema of Trent and the wrath of Sts. Peter and Paul of Quo Primum.

This is the problem with the 1962 Missal, and Father Ratzinger knew this. That is why he was not only OK with the 1962 liturgy, but in "Summorum Pontificuм" he made sure that the Mass of Pope St. Pius V does not return.
Abp. Lefebvre imposed the 1962 liturgy on the SSPX. Did he ever say the new Mass? Did he "ordain" (God forbid!) a priest with a new rite of Order?

By the way, your statement that Fr. Pierre Epiney, who is the primary source of information, is "repeating a lie" is a defamation of a very holy priest, unless you have specific proof to the contrary. Bishop Pierre Roy, ordained by Bp. Williamson and consecrated by Bp. de Silva, who was ordained to the Priesthood by Bishop Williamson in 2017, is a valid Roman Catholic Bishop. We owe him at least the benefit of the doubt that he would not disseminate disinformation.

Casting doubts on the name, character, and reputation of true priests and bishops carries a heavy burden of proof for a lay Catholic.

"It is said in addition that no discussions on restoring unity were ever begun without the acceptance of the prior condition that the commemoration of the Roman pontiff should be included in the sacred liturgy, nor was a union which had been agreed on regarded as complete until the previous condition had actually been put into effect.
The clear result of all this is that the Latin and Greek churches agree in recognizing and affirming that the commemoration implies a profession of due subjection to the Roman pontiff as head of the Church, and of a willingness to remain in the unity of the Church. On the other hand the omission of this commemoration signifies the  intention of steadfastly espousing schism."

The fact is, it is altogether unreasonable to assume +ABL ever said the Mass non-una cuм - to una cuм faithful. OTOH, it is very easy for non-una cuм faithful to believe he did.

Nobody's "assuming" anything.  In fact, the reason this is newsworthy is that pretty much everyone had assumed the opposite.  We're not dealing with assumptions, but with a report from Bishop Roy about what Father Epiney says Archbishop Lefebvre told him.  Again, irrelevant 33 years later, but this has nothing to do with "assuming" anything.

The whole idea that +ABL was at anytime non-una cuм is altogether absurd to the Nth degree.

The idea that he considered it at one time or another is not the issue