And, they weren’t careful about what images they selected from secondary documents, because they accidentally selected some typeset proportional font text and kerned font text (possibly from a magazine of that era, or a later printed document) and included it in their primary editing layer.
If it's true that some of the text consists of proportional and kerned fonts that couldn't have been created in 1960s, then this seems to throw a new wrinkle into things. I don't see how the software processes involved in scanning the image -- the ones that try to recognize text and that create the various layers -- can explain this. Maybe you've covered it already but I don't think I've seen it.
If it were, then it would.
But typewriters had proportional fonts as far back as the 1940s. And there's no evidence of any kerning. Those are simply closely-spaced typewriter strikes. And they're uneven. A typewriter with sophisticated kerning wouldn't produce randomly-uneven text.