My, there is a lot of 'stuff' on this thread. I am 'self taught' but have only been playing for 39 years so I have the 're-learning' experience to look forward to - perhaps. What I have found over the years is that the barre chords played 'properly' works pretty well playing electric in a six piece ceilidh band with two melodeons and sax/brass but for solo or duo/small acoustic work the thumb can come in handy as an extra fretting digit. I can think of some tunes I play in this context, for example 'Sidney Smith's March', where the descending bass line in the third part of the tune utilises the 'wrapover' thumb on both 4th and 5th strings - the 6th being already dropped to D. I can't really figure out another way of doing it. Ultimately, I suppose, you do what's right for you.
Does anyone remember - on the theme of 'self-taught' - a book called 'Play Guitar' by someone called Dan Morgan ? I spent a lot of teenage time with that when I should have been going to parties!!