I don't recall seeing this thread before and I'm certainly not going to wade through it now. However, how do you think traditional singers learned? They learned by listening to their previous generation and accepting whatever criticism came their way. It's worth remembering that song traditions used to be gerontocracies. IE., the older you were, the higher your status within the group. Therefore, it would not have seemed at all out of place for established singers to give free and pointed criticism to younger singers.
Unfortunately, in this case at any rate, we're living in a more egalitarian society, where it would be considered rude to offer such advice, however politely it was intended.
So that's problem number one.
Problem number 2 is that some people just don't want to learn. I've known singers who've been performing for forty odd years and still make the same basic mistakes. Invariably, that is because they don't want to improve and if you ask them they usually say "we're just having a bit of fun". Well, I'm sorry but, if you can't be bothered to give it your best shot, please don't aim it down my lughole!
Against that, I occasionally find singers who are keen to learn, but who maybe lack confidence or haven't resolved the problems of breath or pitch control or whatever.
The way forward is to offer them encouragement and appreciation. Then, when the time is right, say something like "That was great, but wouldn't the audience find it easier if you took a breath at the end of a line instead of in the middle?.
Everybody can sing and sing well. It's just that a lot of people on the singaround circuit are too lazy or inconsiderate to make the best use of whatever talent they've got.