A double agent would not get you "full take", it'd be impossible to hide the traffic. A double agent could maybe feasibly steal keys from Google, but they'd have to do that all the time because the keys are constantly rotated.

And even then, stealing keys does not give you passive decryption and active decryption would be incredibly noisy.

NSA does not have enough money to spend to be able to incentivize Google to give them full take intercepts either.

I think you are not being creative enough with how one might attempt this. For example, splice the cables leading to the datacenter, put an inconspicuous chip in the servers that intercepts the keys and feeds them via wireless signals to a collection point. Perhaps you could even do something clever like put very short range EMF into a metal co-location rack and collect the signals almost totally invisibly using a mesh network of devices built into the metal.

There's lots of fun tricks you can think of when you have national resources at your disposal.

However, you are forgetting that NSA works for Google. It works to support the promotion of American companies worldwide. They're on the same team, and Google knows that. They even have the same mission: To usefully organize the world's information!

Now that Google is openly a military contractor, it's even easier to make this click. Back in the day, you had to read things like this Julian Assuage piece to understand this: https://wikileaks.org/google-is-not-what-it-seems/