I was myself initially troubled about the validity of the Thuc line, and at the the time I did not have the hours of time to devote to the research that would have been required to debunk the SSPV allegations. But once I did sit down and look at the arguments, the SSPV case against the Thuc line holds NO WATER whatsoever, and it was extremely irresponsible. Father Kelly quite literally pulled these made-up principles out of thin air and then found some out-of-context quotes to back them up.
This requirement that "competent" witnesses are required who can attest specifically to whether the matter and form were correctly applied simply does not exist. As long as the minister has been properly trained, e.g. a bishop like Thuc who had been a seminary professor and who had personally consecrated a number of men before Vatican II, the competence of the minister is presumed. During the Cold War, bishops were clandestinely consecrated with no witnesses present (to minimize the risk), and their validity was never doubted by the Church. In fact, +Thuc himself had one of these commissions to consecrate bishops clandestinely ... since he was operating in Communist territory. One could argue, even, that, if there were no legit popes since Pius XII, his permission to consecrate endured ... and one could even make a case that they were done with all necessary jurisdiction.
I mean, what if the priest had botched the Baptism Rite of a man who was being consecrated? Then he wouldn't be a valid bishop. Was there anyone present who could swear that the priest poured water correctly on the infant's head and said the proper Latin formula? Such testimony was never demanded by the Church. Even though it's theoretically possible that it was botched, the Church leaves it to God's providence to take care of such matters.
What if I'm assisting at Mass? Could I please get an altar boy trained in Latin to sign off that the priest got the words of consecration right before I go to receive Holy Communion? According to Father Kelly, I can never receive Holy Communion, then, since I would have to hold the consecration to be doubtful due to lack of sufficient witnesses.
It's utter nonsense.