I want to address your main question, rather than the various subsidiary ones. You asked if it's possible that LIGO's results are anomalies.
No. There's no way that LIGO's results are anomalies.
The most vivid proof is the detection by LIGO of GW170817/GRB170817A, a binary neutron star merger that emitted a gamma ray burst. (Paper here.)
Binary neutron star mergers are incredibly violent explosions. When they occur, the merging systems emit both gravitational and electromagnetic radiation and eject a cloud of matter. The electromagnetic radiation comes in the form of a gamma ray burst. There is no brighter source of electromagnetic radiation in the cosmos.
As gravitational and electromagnetic radiation emerge from the explosion, they interact with the ejected matter. Electromagnetic radiation travels slower in media than in vacuum, in proportion to the amount of matter interacted with and in proportion to the strength of the interaction. This is also true of gravitational radiation, but the gravitational interaction is so much weaker that it can be neglected. Consequently, the gravitational waves escaped the explosion before the electromagnetic ones, and arrived here on Earth first.
On 17 Aug 2017, LIGO detected a large gravitational wave, and then, 2 seconds later, the Fermi gamma-ray telescope detected a gamma ray burst. Two seconds may not seem like much, but it's an eternity for these kinds of signals. The gravitational wave had already passed by the Earth when the gamma rays arrived.
With help from its sister instrument Virgo (which had a known blind spot where the explosion occurred), LIGO was able to triangulate the gravitational wave source well enough to confirm that they were receiving radiation from the same source.
That, to my mind, is a smoking gun. LIGO saw something in the sky, and when other detectors were pointed there, they saw it too.