Yes, it's possible.

Yes, it's being done.

Yes, they have compute capable of analyzing live traffic at those data rates.

The two methods I know of are:

1. Cutting the fiber and installing a splitter. This could be done during a "normal" maintenance window, in cooperation with the fiber owner. Traffic would just flow over another line.

Eg. https://en.wikipedia.org/wiki/Room_641A

2. Leaving the fiber intact, but bending the fiber and reading light emanations. This technique has been done for decades. You'll have negligible attenuation.

Examples of these technique were described in the James Bamford books, Body of Secrets and The Shadow Factory.

https://en.wikipedia.org/wiki/Body_of_Secrets

https://en.wikipedia.org/wiki/The_Shadow_Factory

IIRC, the book Blind Man's Bluff describes a US submarine mission sneaking into a real hairy area (I want to say the Black Sea?) and applying method #2 to some Russian fiber.

https://www.amazon.com/Blind-Mans-Bluff-Submarine-Espionage/...

I'm reminded of an excellent book detailing the US's own interference with Russian cables 30 years back:

https://www.amazon.com/Blind-Mans-Bluff-Submarine-Espionage/...