there can be a very big difference between, "I don't know what happened," and, "It does not matter what happened."

Of course. But I posit the possibility of "It is *impossible* to know what happened." I don't seriously believe that such a condition applies to something as recent as the Cuban Missile Crisis.

Hm. Lots of time travel stories that use the "time is invariant" model also use the notions of "event shadows", periods of time which, being unrecorded, might 'turn out'/'be manipulated' to contain events which work out to one's advantage. An interesting fictional tack might be to take the incredible amounts of obfuscation around, for example, the JFK assassination, as a deliberate attempt to *create* an event shadow...