There's also positive doubt concerning the validity of "Archbishop" Marcel Lefebvre's ordination to the priesthood, which makes the entire SSPX a
Work of Human Hands if I may coin a phrase. But that shouldn't shake anyone's faith of course.

This is false.
No it's not. Lots of people have positive doubt about Lefebvre's ordination. Just use the google, you'll see. Don't shoot the messenger.

My dear Doctor, there aren't any positive doubts in regards to the priestly ordination of Marcel Lefebvre. Cardinal Lienart was never observed to have said that he did not have proper intention. This, combined that he followed the rite, imposed his hands, and said the form means that we must presume (and therefore hold) the ordination as valid.
Even if it is proved beyond a shadow of a doubt that Cardinal Lienart was a luciferian or mason, I don't see any positive doubts.
In the past, many openly and obviously freemasonic and even apostate bishops consecrated and ordained other men, yet the Church did not say that these men needed to be ordained or consecrated sub conditione. I have some examples:
1. The Case of Talleyrand
Before he was Monsieur le Premier Ministre de France, Charles Talleyrand was l'abbe and Monseigneur Talleyrand, Bishop of Autun. He was ordained a priest in 1779, and consecrated a bishop in 1789. The Catholic Encyclopedia notes:
[H]e read the "most revolutionary books", and at length, giving up his priestly life, plunged into the licentiousness of the period. Having, nevertheless, been ordained priest (1779) and appointed general agent of the clergy (1780) he rapidly acquired a reputation as a man of ability. The Assembly of the Clergy of France of 1782 appointed him their promoter, and in 1785 he became secretary. Owing to his notorious immorality he obtained an episcopal see only through a promise wrung from the dying king by his father, Comte Daniel de Talleyrand.
Talleyrand consecrated over ten bishops, and when they left the French Church to return home to the true Church, neither Pius VI nor any other authority required them to be conditionally consecrated. The same goes for those they may have ordained, confirmed, etc.
2. Other bishops in France
Many other bishops in France were shown to be masons. No conditional consecrations were ever demanded by the Church. French historian (and Catholic, R.I.P.) Henri Daniel-Rops said:
“One fact is inescapable: the lodges contained a large number of ecclesiastics... At Caudebec fifteen out of eighty members of the lodge were priests; at Sens, twenty-five out of fifty. Canons and parish priests sat in the Venerable Assembly, while the Cis- tercians of Clairvaux had a Lodge within the very walls of their monastery! Saurine, a future bishop of Strasbourg under Napo- leon, was a governing member of the Grand Orient. We cannot be far from the truth in suggesting that towards the year 1789 a quarter of French freemasons were churchmen... [In 1789 there were] seven atheists and three deists out of one hundred and thirty- five French bishops.”