a) Saying "gravitational waves don't shake the mirrors, it's length contraction" is as confused as saying "the Sun doesn't pull on the Earth, it bends spacetime around it". In each case, you're describing the same mechanism with different words. The whole point of the equivalence principle is that gravitational forces are equivalent to changing the local inertial frame. Ultimately, the Einstein field equations are what they are, but both types of description in words are correct (and inexact).
b) Virgo and GEO600 didn't detect gravitational waves. LIGO did.
"Only the LIGO detectors were observing at the time of GW150914. The Virgo detector was being upgraded, and GEO 600, though not sufficiently sensitive to detect this event, was operating but not in observational mode."
The GW150914 event was the strongest one observed in the period LIGO was operating:
"Detected with ηc = 20.0, GW150914 is the strongest event of the entire search."
So, the strongest event happened to occur when the other detectors were not running.
Also a physicist. I may have let my passion for what the guys at virgo have been up to bias my judgment - visited years ago when they were commissioning the place, and had a long, long discussion about the problems they overcame, mostly around optics and noise, which they shared solutions for with the ligo folks. Now the paper is out I've untwisted my knickers as I can see they've cited the teams around the globe. The media up until that point kinda gave the impression it was a sole effort.
a) Saying "gravitational waves don't shake the mirrors, it's length contraction" is as confused as saying "the Sun doesn't pull on the Earth, it bends spacetime around it". In each case, you're describing the same mechanism with different words. The whole point of the equivalence principle is that gravitational forces are equivalent to changing the local inertial frame. Ultimately, the Einstein field equations are what they are, but both types of description in words are correct (and inexact).
b) Virgo and GEO600 didn't detect gravitational waves. LIGO did.