Tom, here is how you get the proof. Apply to law school, graduate, pass the bar and work some of these cases on behalf of paitents.
This attorney wrote a decent explainer about how it works and why it is bad for you.
https://www.sgrlaw.com/ttl-articles/erisa-litigation-basics/
I worked on these types of cases as a young law clerk. The sad thing was the regular workers at these companies can't afford to file Federal Lawsuits to get their treatment.
It is only when company owners or upper management are wrongfully denied that you see these suits. The insurance companies know this too and they are very good at predicting when they think they can get away denying claims because their victims lack the means to pursue it.
They tend to deny a lot of legitimate claims from the rank and file empoyees and approve almost any claim higher level employees make.