I'm pretty doubtful this even happened. If you search the internet, you will not find any article or public statement by any priest who claims to have been there at this ceremony and to have seen this error with his own eyes. There must have been numerous priests there, and no one has ever written anything denouncing this problem or even pointing out its existence? Really?
Moreover, I've spoken to a lot of old guard SSPX and former SSPX priests (mostly sedevacantists) who were there at that ceremony, and none of them seem to know who saw this error or claims it took place. No priest has ever claimed, even privately as far as I know, to have seen this with his own eyes. No one can tell you, "Oh yeah, Fr. Joe Smith saw Abp. Lefebvre use only one hand in that ceremony." All you get is, "Well, I don't know, the stories say that there was only one hand used."
Again, so many priests watching such an important ceremony, and not one of them claims to have seen Abp. Lefebvre impose only one hand.
Sorry to be chiming in for the first time here on p.13 of this thread, but I didn't know this was the subject matter of this thread from the title.
Doubly sorry therefore for not having read through it all (or hardly any of it, in truth), but aside from the fact that I don't think one hand or two is relevant for validity, I saw this issue raised for the first time a couple/few months ago in another thread, and wrote to Bishop Williamson to sak if he was aware what some people were saying.
This was his email response (excerpt, which I don't think he would mind me posting):
"I have heard it before.The Archbishop was a competent theologian, and scrupulous about ceremonies and ceremonial. Little is less likely than that he would have confused the one hand of the diaconate with the two hands of the priesthood.I would like to see who are the "eyewitnesses" of the one hand....God bless, BpW."