Using the criteria which you propose to judge Lefebvre's innocence (ie. grave fear for the true faith and acting for the salvation of souls rather than disobedience of papal directives), I see no way to conclude that Thuc excommunicated himself with this consecration. Who's to say that Thuc did not truly believe that he was acting in the best interests of the Church and for the salvation of souls? He more than likely did believe this. Yet excommunicated he most surely was.
This issue seems to me to be similar to the "conscience" question. It's often said that if a man's conscience does not accuse him of wrongdoing, then he has done no wrong if he does not know any better and is doing his best to follow his conscience. This is only half the story. The other half is that we have a duty to form our conscience in the light of authentic teaching and truth. So having a clear conscience is not, per se a free pass if we have not taken care to form our consciences with due care and attention. For that matter, it's quite possible that those presently committing atrocities in the name of Islam feel that they are protecting the "true faith".
Similarly here. Just because a bishop, any bishop, feels that a situation calls for the consecration of other bishops and he really believes this, does not give him carte blanche to do it. For implicit in this judgement is the assumption that he and not the Pope has a mandate to do this. It assumes that he and not the Pope is the guardian of the true faith.
Using your criteria, nobody can be judged guilty of flouting papal authority.
Furthermore if you claim that the Pope cannot know if Lefebvre is guilty, then you cannot know if Lefebvre is innocent. You cannot have it both ways.
The two cases are somewhat different. Archbishop Lefebvre acted to preserve the traditional Mass from destruction. He never recounced the Pope himself personally as did Archbishop Thuc. This said, perhaps the latter felt justified in conscience. We are living in terrible times. It is difficult to get at the real truth in these matters. Rome certainly is the last place to look if one wants to get the straight story about anything. It has its own very sharp axes to grind.
Your point about an informed conscience is true--but it is a leap to think that Archbishop Lefebvre, who had spent his lifetime as a missionary priest and bishop in Africa, whose entire lifetime was spent in strict obedience to the faith could not recognize the dangers posed to the Church by the postconciliar onslaught against Catholic Tradition. It is an absurd contention. Rather one has to wonder about postconciliar papal consciences--and how they may be reconciled to the disasters that have been so recklessly imposed on the Church.
One other point about informing one's conscience. Have you ever tried to get clear information from a Vatican source? The entire postconciliar edifice has been predicated upon obfuscation and denial of the principle of non-contradiction, as if past doctrines might be affirmed and denied at the same time. Even the Catechism is riddled with ambiguity. The last thing Rome gives anybody these days is a straight answer.
Archbishop Lefebvre, on the other hand, derived from a Thomistic theological background that prided itself on clear thinking. He was well aware that the doctors of the Church warned even popes must be disobeyed if their commands would be harmful to the Church. And he was fond of citing St. Paul writing to the Galatians: "If anyone--even an angel come down from Heaven--should preach to you a gospel other than the one I have preached to you, let him be anathema."