Why are there giant concrete tunnels in the desert?

Ok I have several questions.
1) How do they know that the two facilities didn’t simply have a similar noise signal at the same time? It seems likely that this would happen frequently over time spans of years.
2) How do they know the gravitational wave came from the colliding black holes? The source cannot be triangulated with fewer than 4 detectors and the error over such a large distance and narrow detector must be massive.
3) Bouncing the laser 300 times along the tube length is trading accuracy for precision. I understand they need better precision to detect such small deltas but how do they account for the lost accuracy?
4) How do they know gravitational waves move at or near the speed of light? Correlating with a coincident astronomical observation is backwards reasoning. If gravity waves were slower then the astronomical event might have happened before recorded history. And if gravity waves are faster then we would not see it yet.
5) Why would waves dissipate in magnitude like light? If spacetime is expanding then wouldn’t the gravity waves dissipate faster?
6) how do they know the distance to the blackhole? Parallax has error. Standard candles have error. Red shift has error. Black holes do not emit. Inference of distance based on other objects behind blackholes has error. Is all that error accounted for in the gravitational wave analysis?
7) how do they account for the orientation of the detectors? A gravity wave parallel to one of the shafts would have a much larger distortion than a gravity wave perpendicular to the shaft. So the magnitude of the signal would be dependent on orientation of the detector rather than the magnitude and distance of the source.

Please let me know if any of these issues have been addressed