As always, this may be data you've already found, but just in case not:
Though NTSB does not have CAB archives prior to about 1965, there is a section of CAB Records in the National Archives at College Park, MD, details at
https://www.archives.gov/research/gu...roups/197.html
197.3.5 Records of the Office of the General Counsel references "aircraft accident investigations, 1952-64", as well as some contemporaneous records related to liability and litigation which, though a longer shot, might include pertinent material.
By the way, concerning the existence of retained records, I noticed you wrote above in one post,
"...no one gains a thing by releasing them to me, or acknowledging to me that they exist. There's always the possibility of liability issues. A reopening of the investigation would shake them loose, of course."
As someone whose work involves quite a lot of research, I think you are right to be cynical...but I think you're cynical about the wrong thing.
Realistically, none of these agencies or companies has anything to lose by releasing archived records from a closed, 55-year-old, well-publicized public investigation. But I began my research from a point of cynicism as to whether some of the people you contacted for information really knew what they were talking about. Or if they pointed you in another direction just because they didn't want to be bothered. Both are a lot more likely than them trying to hide something or stonewall you.
To be fair, I'm sure the intentions of most were good, and they genuinely believed they were giving you accurate information. But as you've seen, some of those records exist in public archives despite assertions to the contrary.
Loss of institutional knowledge is endemic, especially after half a century. Most likely there's just no one left at the agencies/companies or their successors who has first-hand knowledge of where old records ended up. And there's more stuff out there no doubt in collections of personal papers and correspondence of the various executives and investigators. The Chicago Tribune now sells old photos from its archives, so you may be able to get a clear print of the track debris photo...albeit not for free. Lockheed's website contains references to all documents from California being moved to their warehouses in Marietta, GA when the CA facilities closed. Someone must know what's in those warehouses, given the number of scholars, researchers, and scientists still writing about Lockheed's role in aviation history, and those interested in every aspect of the various Electra versions. Etc., etc., etc.
In other words, your sister is right. Don't give up. We have not yet begun to search :-)