I don't know the validity of the original stories (although the second one is certainly VERY believable!) but I do know that instructor (or divemaster) training makes you think about a lot of things you might not otherwise have remembered, when you are working with students. Those of us who have dived for a long time may not remember that we had to LEARN to equalize (something which is given short shrift even in professionally run classes sometimes). The choice of dive site for students is important, and having a hard deck within recreational range is really pretty necessary.
There is a lot of learning about how to be proactive and mitigate risk, in the teaching of diving. I do think a very thoughtful person can probably mentor a friend through basic open water safely, but I think it is far better done by people who have had specific education in doing it.