I rather like this. It's the sort of thing that some of us have been saying would bring the gap towards making the inevitable "first couple years of the '60s" context make sense. (I suppose Vietnam is involved because the French somehow made off with Excalibur, or knowledge of it, at some point?)
I'm concerned about the prominence of Kennedy, just because of how it might deal a blow of unreality; remember, at the very least, things usually look how we expect them to look. Would a Kennedy lookalike/soundalike be enough for the extent that he's needed? It's neither a total cameo, nor a part so substantial that an actor would be able to establish himself firmly in the part.
But, of course: that's sort of a silly concern. Conceptually, this is quite nice. And I've always liked the Indy being caught between the Reds and his own government angle for a Cold War story, with neither side deserving the artifact. Question: anyway that it would make sense for Indy to be responsible for destroying/getting rid of Excalibur? We haven't had that precise version of the loss yet, but we had it threatened the very first time around.