One thing that helps to make this unlikely, is that monero is forked from a scam. If the nsa was behind it than the scam would have been far more clever than the scam that was bytecoin. I.E. the scam would have been things like what you are talking about, clever backdoors and what not. And they wouldnt have risked hurting the adoption of a crypto that had subtle back doors engineered into it by creating a fake blockchain and spoofing dates on pdf's and what not.
You could say well yea but they are just playing chess on another level and anticipated that people would make the argument i just made, but thats really really getting into tin foil hat territory. Even for people like us who tend to be relatively inclined to believe in conspiracies.