Archbishop Lefebvre was ‘non Una Cum’ in his last years

False: See conference of April 1, 1989 to the Dominican nuns in Brenne, where he ridicules those adopting the non una cum position:

Archbishop Lefebvre - Against Sedevacantism

Could it perhaps have been because he received a letter purportedly from Paul VI in 1984 when the rest of the world believed that the Pope of that name had died?

Some people hold to the view that Pope Paul VI did not die in 1978. But, having been bullied, forcibly imprisoned and replaced by an Impostor, eventually, broke free of his confinement in 1972, Certain evidence was gained by Private Detectives to support this belief, which is covered in a Book called "The Broken Cross" (see link below)

In 1984 at the side of a road, Fr Bonaventure Meyer recorded on audio a conversation that he had with Archbishop Lefebvre, during which the Abp discussed quite cheerfully the topic of a note that had been handed to him in 1984 signed by "Paul VI"which asked him to "Consecrate many Bishops" On the audio Abp Lefebvre says that the letter is carried with him wherever he goes.

Maybe it was because he had received that note from a person claiming to be Paul VI in 1984 - when Paul should have been dead and buried, that Archbishop Lefebvre Consecrated those 4 Bishops In 1984 and wouldn't offer a Mass in communion with a potentially false Pope ?
It would be interesting to know where that letter is now.... SOMEBODY must have it?

Here is the audio of Abp Lefebvre speaking of the note - in French
Mgr Lefebvre témoigne du mandat de Paul VI

Here also is the same audio recording contained in a longer video -in French but with English subtitles. The letter is discussed and the audio can be heard about 1 hour 28 minutes or so into the video. (about 7 minutes from the end)

PAUL VI And The Mystery Of Iniquity (1/4)

A link to the Book "The Broken Cross" by Piers Compton.

The Broken Cross : Piers Compton : Free Download, Borrow, and Streaming : Internet Archive

This is supported by comments made by the Archbishop and Fr. Joseph Bisig (a former SSPX member and co-founder of the FSSP).

From The Angelus, July 1986:

“You know, for some time, many people, the sedevacantists, have been saying, ‘there is no more pope’. But I think that for me it was not yet the time to say that, because it was not sure, it was not evident…” (Archbishop Lefebvre)

“I don’t know if the time has come to say that the pope is a heretic … Perhaps after this famous meeting of Assisi, perhaps we must say that the pope is a heretic, is apostate. Now I don’t wish yet to say it formally and solemnly, but it seems at first sight that it is impossible for a pope to be formally and publicly heretical. … So it is possible we may be obliged to believe this pope is not pope.” (Archbishop Lefebvre)

From Catholic Register, November 2018:

“Pope Paul VI’s subsequent suspension led to a change in Lefebvre’s attitude towards Rome, and his language became increasingly “polemical.” Lefebvre began to entertain sedevacantism, the idea that Paul VI was not the real pope, and thus the Chair of Peter was vacant.” (Fr. Bisig)