Despite the location, there was probably more evidence about this accident than any other. They even managed to recover the units that the waypoints were manually punched into. They were analysed in the US and they were able to determine that the waypoints entered were those on the flight plan.
The task was not too great for a High Court judge. He had a year, with nothing else to do. Go and read some other High Court judgments. Many of them are much more complicated. The problem was that Mahon misinterpreted some evidence, ignored other relevant evidence, and was deliberately misled by the union, particularly as regards the critical evidence of Captain Simpson. He then decided to finish his report with the "orchestrated litany of lies", which was extremely foolish and consistent with someone suffering from a neurological condition. Note that he died two years later of a head tumour.