You've raised some good points, IPUK, which help underscore the many problems with the LAD legend. After beating my head against the wall for years, I long ago entertained the notion that the LAD might be something entirely different than what people have assumed the past 150 years. What that 'something' was/is is an open question. Let me float a theory: the Lost Adams Diggings was/is a coded message intended to mask an as yet unknown event dealing with a large amount of gold within the trappings of a 'lost mine' story. Let's assume that the key details, characters and landmarks in the story are for
recognition value only - red flags to attract attention, if you will.
Super, Springfield!
Thinking outside the box in an unorthodox manner without accepting the 'normal' version(s) of the story, it all starts showing promise. Add to you comment, that there was the possibility that Snively, Adams, Brewer and Davidson may have served together in the Civil War and been discharged together, and things start to take some sort of shape. With Purcell's research, you also have the Latter Day Saints' connection and goings-on across the border in old Mexico. Only Adams stuck to trying to raise finance and expeditions to try and recover whatever it was that had been hidden all those years before. There is simply too many divergent strands and evidence to suggest that it was all a hoax and a figment of Adams' imagination. What might not be true is certain elements and timeframes and players, but there was an incident, there was gold involved, it happened sometime in the 1860s and it was in a lonely place without many witnesses.
This might be one of those occasions when Sherlock Holmes might say "When you have eliminated all which is impossible, then whatever remains, however improbable, must be the truth?"