An important point about all this (which I guess is what AdamCH is saying in some of his posts) is that what you are describing as 'documentary evidence' is, in reality, usually merely a statement written by the person who undertook the work. It's therefore not really very strong as 'evidence', even if written by a 'professional', since it could be complete fiction, particularly in the case of a 'cowboy'. The cowboy (or you or I) could, for example, write down a set of test results which were totally credible, but also totally fictional.I'll give you that one, providing you can prove with documented evidence, that the installation and fabric of the build was no worse than when you started the work. ....
In the absence of third-party inspection, I don't see how this problem can be avoided - and the thing which AdamCH doesn't say is that it applies equally whether the documentattion is written by decent electricians, 'cowboys' or DIYers.
Kind Regards, John