By a contradiction which shows how little the Tribunal were convinced of the justice of their own sentence, they granted hera declared schismatic and hereticthe privilege of Holy Communion, which all these long months had been denied her.
I will grant you that the Tribunal attempted to excommunicate her; it would seem, however, that it was invalid (and hence there was no excommunication) since it was manifestly contrary to the law. As the Catholic Encyclopedia says:
The first trial had been conducted without reference to the pope, indeed it was carried out in defiance of St. Joan's appeal to the head of the Church. Now an appellate court constituted by the pope, after long inquiry and examination of witnesses, reversed and annulled the sentence pronounced by a local tribunal under Cauchon's presidency. The illegality of the former proceedings was made clear ...
In any case, the example of St. Joan doesn't seem to prove your point. Are you arguing that since she was truly excommunicated unjustly, the excommunication had no force, and hence the same is true of Msgr. Lefebvre and his bishops? St. Thomas explains why whether or not she was excommunicated unjustly or not excommunicated due to illegality of the sentence, she would not have been damned. For he says, as I pointed out in my previous post:
In this case, if the error, on the part of the sentence, be such as to render the sentence void, this has no effect, for there is no excommunication; but if the error does not annul the sentence, this takes effect, and the person excommunicated should humbly submit (which will be credited to him as a merit), and either seek absolution from the person who has excommunicated him, or appeal to a higher judge. If, however, he were to contemn the sentence, he would "ipso facto" sin mortally. (Sup., q. 21 a. 4)
Obviously the case of St. Joan, who appealed the sentence and submitted, is far different from the case of Msgr. Lefebvre and his bishops, who did and do in fact contemn the sentence against them. Hence no comparison is possible, it would seem.
Thank-you for helping me make my point.