What is the basis for the conclusion that every Russian/Cuban embassy around the world including those Mexico City was under 24/7 surveillance back in the early 60s?
The open secret about embassies is that they're Spy bases. So it's reasonable to assume that they all document and photograph every person who visits their enemies as well as documenting visitors to the embassies of rival nations.
They're always being surveilled by someone and all diplomats know that.
With all that said, it's very telling that there are no photos of the real Lee Oswald visiting either the Soviet or Cuban embassy. It's shocking that even the Soviets and Cubans don't have surveillance photos of him.
The spy bases in Mexico City were all asleep that weekend I guess. Or maybe the real Oswald never visited them. We don't know enough unfortunately to conclusively say it was him.
Even if true, isn't it entirely possible that the CIA did capture an image or recording of Oswald and decide after his death that it wasn't worth revealing the extent of their surveillance methods to the Russian/Cubans?
No, that's not a reasonable excuse. They didn't need to release the surveillance information or photos to the public. If they had photos of the real Oswald, they could've been shown to the President, J Edgar Hoover, and others with top secret security clearances. As far as we know, that didn't happen. In fact, Hoover implied that someone might've impersonated Oswald in Mexico City in one of his conversations with LBJ.
At 10:00 am on Saturday, November 23, President Johnson asked FBI Director Hoover if there was anything new concerning Oswald’s visit in Mexico City (it’s unclear when Johnson first had learned of the Mexico City visit). It was at this point – just 22 hours after the assassination– that Hoover told Johnson about the Kostikov link and that it was not Oswald’s voice on the tape; he had been impersonated.
https://www.pbs.org/wgbh/frontline/article/oswald-the-cia-and-mexico-city/
The Warren Report didn't take a deep dive into Oswald's activities in Mexico City. Much of what we know wasn't made public until the HSCA investigation.
These are very secretive paranoid types. They are very rarely going to release any such information absent a very good reason.
I don't think covering up incompetence is a good reason. I don't think covering up a potential conspiracy is a good reason.
I don't assume good reasons for lies or intelligence coverups. That extends to stuff like the 2012 Benghazi attacks. Another CIA coverup.
With Oswald dead, his visit to Mexico City known, and all the evidence pointing toward his guilt, there would have been very little incentive to release any such photo or recording.
If there was a good faith effort to investigate a potential conspiracy, they wouldn't have covered that stuff up.
The truth is, Johnson and others didn't want to open a can of worms on the question of a potential conspiracy involving the Soviets or Cubans so they shut down inquiries into the Mexico City stuff.
Assuming Oswald was a lone assassin was the least geopolitically challenging explanation.