With respect, I think you're missing the point. I agree with everything you say above, BUT IMO, the chances that a person who comes to this forum to ask how to change a socket or light switch will read books about testing and hire an MFT in order to do the job are close to negligible - do you disagree?I am not saying only qualified electricians should be allowed to do electrical installation, just that there is no justification for saying there should be different requirements for DIY vs professional. I am not saying they need a full set of quals, but should understand the basics. For testing, there are books and hire shops. You don't need 239x to understand the significance of the readings, just knowledge of what readings to expect. The minimum would be a loop and RCD test.
Realistically, we EITHER have to accept that the people I'm talking about will not do any significant testing OR tell them they shouldn't do the job. To pretend to ourselves that they will acquire the knowledge/kit and do the testing is really (again, IMO) a totally unrealistic attempt to comfort ourselves.
Kind Regards, John