Also, everyone is convinced that their proxy is the best achievable, although of course they test it only against false positives, not false negatives.

> everyone is convinced that their proxy is the best achievable

In my experience, people merely hoped their process was good enough. And it usually was. The "we only hire the best" is marketing propaganda and they know it.

Yeah. The best proxy is just to hire somebody as a temp contractor for a month or so if both parties can afford it.